Scientifica (Cairo). 2012; 2012: 215308.
Published online 2012 Dec 12. doi: 10.6064/2012/215308
PMCID: PMC3692299
NIHMSID: NIHMS474799
PMID: 23807906
B Cells in Autoimmune Diseases
Author information Article notes Copyright and License information PMC Disclaimer
Abstract
The role of B cells in autoimmune diseases involves different cellular functions, including the well-established secretion of autoantibodies, autoantigen presentation and ensuing reciprocal interactions with T cells, secretion of inflammatory cytokines, and the generation of ectopic germinal centers. Through these mechanisms B cells are involved both in autoimmune diseases that are traditionally viewed as antibody mediated and also in autoimmune diseases that are commonly classified as T cell mediated. This new understanding of the role of B cells opened up novel therapeutic options for the treatment of autoimmune diseases. This paper includes an overview of the different functions of B cells in autoimmunity; the involvement of B cells in systemic lupus erythematosus, rheumatoid arthritis, and type 1 diabetes; and current B-cell-based therapeutic treatments. We conclude with a discussion of novel therapies aimed at the selective targeting of pathogenic B cells.
자가 면역 질환에서 B 세포의 역할은
자가 항체의 잘 확립된 분비,
자가 항원 제시 및 이에 따른 T 세포와의 상호 작용,
염증성 사이토카인의 분비,
이소성 배아 센터의 생성 등 다양한 세포 기능을 포함합니다.
이러한 메커니즘을 통해
B세포는 전통적으로
항체 매개로 간주되는 자가면역질환과
일반적으로 T세포 매개로 분류되는 자가면역질환 모두에 관여합니다.
B세포의 역할에 대한 새로운 이해는
자가면역질환 치료를 위한 새로운 치료 옵션을 열었습니다.
이 백서에서는
자가 면역에서
B 세포의 다양한 기능,
전신성 홍반성 루푸스,
류마티스 관절염,
제1형 당뇨병에 대한 B 세포의 관여,
그리고 현재 사용되고 있는 B 세포 기반 치료법에 대한 개요를 소개합니다.
마지막으로
병원성 B 세포의 선택적 표적을 겨냥한
새로운 치료법에 대한 논의로 마무리합니다.
1. Introduction
Traditionally, autoimmune disorders were classified as T cell mediated or autoantibody mediated. However the improved understanding of the complexity of the immune system has significantly influenced the way we view autoimmune diseases and their pathogeneses. Reciprocal roles of T-cell help for B cells during adaptive immune responses and B-cell help in CD4+ T-cell activation are being increasingly recognized. The observation that most autoantibodies in traditionally autoantibody-mediated diseases are of the IgG isotype and carry somatic mutations strongly suggests T-cell help in the autoimmune B-cell response. Likewise B cells function as crucial antigen presenting cells in autoimmune diseases that are traditionally viewed as T cell mediated. This paper will discuss the role of B cells in autoimmune diseases; however, it needs to be emphasized that most autoimmune diseases are driven by a dysfunction in the immune network consisting of B cells, T cells, and other immune cells.
전통적으로
자가면역질환은
T세포 매개성 또는
자가항체 매개성 질환으로 분류되었습니다.
그러나
면역 체계의 복잡성에 대한 이해가 향상되면서
자가면역 질환과 그 병원체를 바라보는 시각이 크게 달라졌습니다.
적응 면역 반응 중
B세포에 대한 T세포의 도움과
CD4+ T세포 활성화에 대한 B세포의 도움의 상호 역할이
점점 더 많이 인식되고 있습니다.
전통적으로
자가항체 매개 질환에서
대부분의 자가항체가 IgG 동형이고
체세포 돌연변이를 가지고 있다는 관찰은
자가면역 B세포 반응에서
T세포의 도움을 강력하게 시사합니다.
마찬가지로
B세포는
전통적으로 T세포 매개 질환으로 간주되는 자가면역 질환에서
중요한 항원 제시 세포로 기능합니다.
이 논문에서는
자가면역질환에서 B세포의 역할에 대해 논의할 예정이지만,
대부분의 자가면역질환은
B세포,
T세포 및
기타 면역세포로 구성된 면역 네트워크의 기능 장애로 인해
발생한다는 점을 강조할 필요가 있습니다.
2. B-Cell Functions in Autoimmunity
Different functions of B cells can contribute to autoimmune diseases (Figure 1):
secretion of autoantibodies;
presentation of autoantigen;
secretion of inflammatory cytokines;
modulation of antigen processing and presentation;
generation of ectopic GCs.
B 세포의 다양한 기능이 자가 면역 질환의 원인이 될 수 있습니다(그림 1):
- 자가 항체 분비
- 자가 항원 제시
- 염증성 사이토카인의 분비;
- 항원 처리 및 제시의 조절
- 이소성 GC의 생성.
(a) B cells in autoimmune diseases. B cells have antibody-dependent and antibody-independent pathogenic functions. Secreted autoantibodies specific to receptors or receptor ligands can activate or inhibit receptor functions. Deposited immune complexes can activate complement and effector cells. Autoantibodies can bind to basic structural molecules and interfere with the synthesis of structural elements and facilitate the uptake of antigen. Independent of antibody secretion B cells secrete proinflammatory cytokines, support the formation of ectopic GCs, and serve as antigen presenting cells. Both secreted autoantibodies and BCR on B cells can modulate the processing and presentation of antigen and thereby affect the nature of presented T-cell determinants. (b) Pathogenic effects of deposited immune complexes. The Fc portion of antibodies in immune complexes can be bound by C1q of the classical complement pathway, which eventually leads to the release of C5a and C3a. These anaphylatoxins promote release of proinflammatory cytokines and serve as chemoattractants for effector cells. Moreover they induce the upregulation of activating FcR on effector cells. Binding of the Fc portion of the antibodies to FcR leads to activation of effector cells and further release of proinflammatory cytokines and proteolytic enzymes, mediators of antibody-dependent cell-mediated cytotoxicity (ADCC). (c) Effect of antibodies and antigen-specific B cells on antigen uptake. Left panel: antigen bound by antibody is taken up via FcR on APCs such as dendritic cells or macrophages. After processing, antigen is presented on MHC molecules. This FcR-mediated antigen uptake is more efficient than antigen uptake by pinocytosis. Right panel: antigen binds to the BCR of antigen-specific B cells and is internalized. B cells are highly efficient APCs in situations of low antigen concentrations. (d) Effect of antibodies and antigen-specific B cells on antigen processing and presentation. BCR-mediated antigen uptake can influence antigen processing and the nature of MHC-displayed T-cell determinants. Likewise, antigen/antibody complexes are bound by the FcR of APCs and processed in a unique fashion dependent on the epitope specificity of the bound antibody. The BCR or antibody can shield certain protein determinants from the proteolytic attack in endocytic compartments (represented as scissors in this figure). Presentation of some determinants may thereby be suppressed, while others are boosted. Thereby cryptic pathogenic peptides may be presented and stimulate autoreactive T cells.
(a) 자가 면역 질환의 B 세포.
B 세포는 항체 의존적 및 항체 독립적 병원성 기능을 가지고 있습니다.
수용체 또는 수용체 리간드에 특이적으로 분비되는 자가 항체는 수용체 기능을 활성화하거나 억제할 수 있습니다. 침착된 면역 복합체는 보체 및 이펙터 세포를 활성화할 수 있습니다. 자가 항체는 기본 구조 분자에 결합하여 구조 요소의 합성을 방해하고 항원의 흡수를 촉진할 수 있습니다. 항체 분비와 무관하게 B 세포는 염증성 사이토카인을 분비하고 이소성 GC의 형성을 지원하며 항원 제시 세포로 작용합니다. B 세포에서 분비되는 자가항체와 BCR은 모두 항원의 처리 및 제시를 조절하여 제시된 T 세포 결정 인자의 특성에 영향을 줄 수 있습니다.
(b) 침착된 면역 복합체의 병원성 효과.
면역 복합체에서 항체의 Fc 부분은 고전적인 보체 경로의 C1q에 결합할 수 있으며, 이는 결국 C5a 및 C3a의 방출로 이어집니다. 이러한 아나필라톡신은 염증성 사이토카인의 방출을 촉진하고 이펙터 세포의 화학 유인제 역할을 합니다. 또한 이펙터 세포에서 활성화된 FcR의 상향 조절을 유도합니다. 항체의 Fc 부분이 FcR에 결합하면 이펙터 세포가 활성화되고 항체 의존성 세포 매개 세포 독성(ADCC)의 매개체인 전 염증성 사이토카인과 단백질 분해 효소가 추가로 방출됩니다.
(c) 항체와 항원 특이적 B 세포가 항원 흡수에 미치는 영향.
왼쪽 패널: 항체에 결합된 항원은 수지상 세포 또는 대식세포와 같은 APC에서 FcR을 통해 흡수됩니다. 처리 후 항원은 MHC 분자에 제시됩니다. 이러한 FcR 매개 항원 흡수는 피노세포증에 의한 항원 흡수보다 더 효율적입니다. 오른쪽 패널: 항원이 항원 특이적 B 세포의 BCR에 결합하여 내재화됩니다. B 세포는 항원 농도가 낮은 상황에서 매우 효율적인 APC입니다.
(d) 항체와 항원 특이적 B 세포가 항원 처리 및 표현에 미치는 영향.
BCR 매개 항원 흡수는 항원 처리와 MHC 표시 T세포 결정 인자의 특성에 영향을 미칠 수 있습니다. 마찬가지로 항원/항체 복합체는 APC의 FcR에 의해 결합되고 결합된 항체의 에피토프 특이성에 따라 고유한 방식으로 처리됩니다. BCR 또는 항체는 특정 단백질 결정 인자를 세포 내 구획(이 그림에서 가위로 표시됨)에서의 단백질 분해 공격으로부터 보호할 수 있습니다. 따라서 일부 결정 인자의 발현은 억제되고 다른 결정 인자의 발현은 촉진될 수 있습니다. 따라서 암호화된 병원성 펩타이드가 제시되어 자가 반응성 T 세포를 자극할 수 있습니다.
These functions will be discussed in detail below.
2.1. Autoantibodies in Autoimmune Diseases
Autoantibodies can be detected in many autoimmune diseases. Their presence in the peripheral circulation and relative ease of detection makes them preferred markers to aid in diagnosis and prediction of autoimmune disorders. In some autoimmune diseases, the autoantibodies themselves have a pathogenic effect, as will be discussed in the following.
자가항체는 많은 자가면역 질환에서 검출될 수 있습니다. 말초 순환계에 존재하고 상대적으로 쉽게 검출할 수 있기 때문에 자가 면역 질환의 진단 및 예측에 도움이 되는 마커로 선호됩니다. 일부 자가면역 질환에서는 자가항체 자체가 병원성 효과를 나타내기도 하는데, 이에 대해서는 아래에서 설명합니다.
2.1.1. Deposition of Immune Complexes and Inflammation (Figure 1(b))
The deposition of immune complexes composed of autoantibodies and autoantigens is a prominent feature of several autoimmune diseases, including systemic lupus erythematosus, cryoglobulinemia, rheumatoid arthritis, scleroderma, and Sjögren's syndrome. The immune complexes can trigger inflammation through activation of complement and Fc-receptor-dependent effector functions [15]. In the classical complement cascade, the Fc portion of the antibody is bound by complement component C1q, which eventually triggers the activation of the anaphylatoxins C5a and C3a. C5a and to a lesser degree C3a attract effector cells such as neutrophils and NK cells and stimulate the release of proteolytic enzymes and inflammatory cytokines. Activation of complement has been consistently demonstrated in experimental models of immune-complex diseases and in the kidneys of patients with systemic lupus erythematosus and lupus nephritis [16]. The immune complexes can also directly bind to Fc-receptors on effector cells leading to antibody-dependent-cell-mediated cytotoxicity (ADCC).
자가 항체와 자가 항원으로 구성된
면역 복합체의 침착은
전신성 홍반성 루푸스,
냉동 글로불린 혈증,
류마티스 관절염,
경피증,
쇼그렌 증후군을 비롯한 여러 자가 면역 질환의 두드러진 특징입니다.
면역 복합체는
보체 및 Fc 수용체 의존성 이펙터 기능의 활성화를 통해
염증을 유발할 수 있습니다[15].
고전적인 보체 캐스케이드에서
항체의 Fc 부분은 보체 성분 C1q에 결합하여
결국 아나필라톡신 C5a 및 C3a의 활성화를 촉발합니다.
C5a와 C3a는
호중구 및 NK 세포와 같은 이펙터 세포를 유인하고
단백질 분해 효소 및 염증성 사이토카인의 방출을 자극합니다.
보체의 활성화는
면역 복합 질환의 실험 모델과
전신성 홍반성 루푸스 및 루푸스 신장염 환자의 신장에서 일관되게 입증되었습니다 [16].
면역 복합체는 또한
이펙터 세포의 Fc 수용체에 직접 결합하여
항체 의존성 세포 매개 세포 독성(ADCC)을 유발할 수 있습니다.
2.1.2. Stimulation and Inhibition of Receptor Function
Autoantibodies can affect receptor function with different outcomes as illustrated by autoantibodies targeting the thyroid stimulating hormone (TSH) receptor. TSH receptor autoantibodies in Graves' disease stimulate receptor function, triggering the release of thyroid hormones and development of hyperthyroidism [17], while TSH receptor autoantibodies in autoimmune hypothyroidism block the binding of TSH to the receptor [18]. Inhibitory autoantibodies are also found in Myasthenia gravis, where autoantibodies bind to the nicotine ACh receptors (AChRs) and block neurotransmission at the neuromuscular junction, inducing symptoms such as muscle weakness and fatigue [19], and in multifocal motor neuropathy, where autoantibodies bind to the ganglioside GM1 and cause motor neuropathy with conduction block at multiple sites [20]. Other autoantibodies can bind receptor ligands, preventing their binding to the receptor, as seen in Graves' disease with anti-TSH autoantibodies [21]. Table 1 summarizes other examples of receptor autoantibodies, their targets, pathogenic mechanisms, and associated diseases.
자가항체는
갑상선 자극 호르몬(TSH) 수용체를 표적으로 하는 자가항체에서 볼 수 있듯이
수용체 기능에 영향을 미쳐
다양한 결과를 초래할 수 있습니다.
그레이브스병의 TSH 수용체 자가항체는
수용체 기능을 자극하여
갑상선 호르몬의 방출과 갑상선 기능 항진증을 유발하는 반면[17],
자가면역성 갑상선 기능 저하증의 TSH 수용체 자가항체는
수용체에 대한 TSH의 결합을 차단합니다[18].
억제성 자가항체는
니코틴 ACh 수용체(AChR)에 결합하여
신경근 접합부에서 신경 전달을 차단하여
근력 약화 및 피로 등의 증상을 유발하는 중증 근무력증과
자가항체가 강글리오사이드 GM1에 결합하여
여러 부위에서 전도 차단으로 운동 신경병을 유발하는
항-TSH 자가항체가 있는 그레이브스병에서 볼 수 있듯이
다른 자가항체는 수용체 리간드와 결합하여
수용체와의 결합을 방해할 수 있습니다 [21].
수용체 자가항체의 다른 예,
표적, 병원성 메커니즘 및 관련 질병이 요약되어 있습니다.
Table 1
Examples for receptor autoantibodies.
Targeted receptorMechanismAssociated diseaseReferences
| Endothelial receptor type A (ETAR) | Activation | Pulmonary arterial hypertension (PAH) | [1] |
| Angiotensin II receptor (AT1R), ETAR | Activation | Systemic sclerosis | [2] |
| AT1R | Activating | Preeclampsia | [3–5] |
| α 1-adrenergic receptors (α 1-ARs) | Activating | Refractory hypertension | [3, 6, 7] |
| β 1-adrenergic receptor | Activation | Dilated cardiomyopathy (DCM), Chagas' disease | [8, 9] |
| N-methyl-D-aspartate receptor (NMDAR) | Activation | SLE | [10] |
| Glutamate receptor | Activation | SLE | [11] |
| Insulin receptor | Inhibition | Autoimmune hypoglycemia | [12] |
| Muscarinic type 3 receptor | Internalization | Sjögren's syndrome | [13] |
| NMDAR | Internalization | Anti-NMDA receptor encephalitis | [14] |
2.1.3. Facilitation of Antigen Uptake (Figure 1(c))
Autoantibodies facilitate antigen uptake by antigen presenting cells (APCs). Antigen complexed with antibodies is taken up via Fc receptors (FcRs) present on monocytes and dendritic cells [22]. This mechanism is more efficient than pinocytosis and results in 10–100-fold lower necessary antigen concentration for successful T-cell stimulation [23–26]. The importance of this mechanism has been demonstrated in a number of animal studies, where antibodies to various antigens enhanced T-cell responses to the respective antigens [27–29]. Autoantibodies can therefore break tolerance of normal T cells through their capacity to promote uptake of self-antigen by APCs via their FcRs. Indeed, autoantibodies to thyroid self-antigens dramatically enhanced uptake of thyroid peroxidase (TPO) by APCs and subsequent activation of TPO-reactive T cells [30] and blockade of FcγR markedly reduced this response [31]. Autoantibodies have also been demonstrated to facilitate the uptake of myelin by macrophages, and the removal of the Fc-portion of the antibodies prevented antigen uptake [32]. Moreover, FcγR–deficient DBA/1 mice were protected from myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE), suggesting that FcR-mediated uptake of antibody-bound myelin is involved in the pathogenesis of multiple sclerosis [33]. Autoantibody-mediated antigen uptake may therefore be a critical mechanism in the pathogenesis of T-cell-mediated autoimmune diseases.
Further support for autoantibody-mediated antigen uptake as a pathogenic mechanism in autoimmunity comes from an elegant study by Harbers et al. where transgenic mice expressed ovalbumin (OVA) as “self” in both their thymus and pancreatic beta cells [34]. Presentation of OVA by dendritic cells to diabetogenic CD8+ OVA-reactive T cells was significantly stimulated by administration of antibodies specific to OVA. This response was not observed in mice lacking activating FcγR, indicating that the antibody-driven effector T-cell activation was indeed FcγR dependent.
However, autoantibodies are not always damaging to the organism, but can have protective functions [35, 36], and natural autoantibodies are commonly found in healthy individuals. Most of these antibodies are of the IgM isotype and have been speculated to have protective functions. One of these functions is the clearance of dying and aging cells and in mice natural IgM autoantibodies bind to epitopes specifically expressed on apoptotic cells [37, 38] enhancing the clearance of these cells, which may otherwise elicit a pathogenic autoimmune response [39, 40]. Lack of secreted IgM has been shown to correlate with an increase in pathogenic IgG autoantibodies and autoimmune disease possibly due to the lack of removal of apoptotic cells [41–43].
The mouse natural autoantibodies that arise without external antigen exposure are secreted from a subset of B cells, named B1 cells [44, 45], and a similar B-cell subset has been recently identified in humans [46]. In patients with SLE, higher levels of IgM associated with apoptotic cell clearance correlate with lower disease activity [47, 48], and healthy twins of SLE patients often present higher levels of these autoantibodies [49]. Another mechanism of protection by natural autoantibodies is the blockage of pathogenic autoantibodies to react with self-antigen [50], and titers of natural IgM specific to dsDNA correlated inversely with the severity of glomerulonephritis (GN) in SLE [51, 52].
Besides producing antibodies, activated B cells are also fundamental for coordinating T-cell functions as B-cell-depleted mice exhibit a dramatic decrease in numbers of CD4+ and CD8+ T cells, and a significant inhibition of memory CD8+ T cells [53, 54]. There are several antibody-independent mechanisms by which B cells can affect T cells and other immune cells as will be discussed below.
자가항체는
항원 제시 세포(APC)에 의한 항원 흡수를 촉진합니다.
항체와 복합된 항원은
단핵구 및 수지상 세포에 존재하는 Fc 수용체(FcR)를 통해 흡수됩니다[22].
이 메커니즘은
피노사이토시스보다 효율적이며
성공적인 T세포 자극에 필요한 항원 농도를 10~100배 낮출 수 있습니다[23~26].
이 메커니즘의 중요성은
다양한 항원에 대한 항체가 해당 항원에 대한 T세포 반응을 강화하는
여러 동물 연구에서 입증되었습니다 [27-29].
따라서
자가항체는
FcR을 통해 APC의 자가 항원 흡수를 촉진하는 능력을 통해
정상 T 세포의 내성을 깨뜨릴 수 있습니다.
실제로
갑상선 자가 항원에 대한 자가 항체는
APC에 의한 갑상선 퍼옥시다제(TPO)의 흡수를 극적으로 증가시켰고,
이후 TPO 반응 T 세포의 활성화[30]와 FcγR의 차단은
이러한 반응을 현저하게 감소시켰습니다[31].
자가 항체는 또한
대식세포에 의한 미엘린의 흡수를 촉진하는 것으로 입증되었으며,
항체의 Fc 부분을 제거하면
항원 흡수가 방지되는 것으로 나타났습니다 [32].
또한,
FcγR 결핍 DBA/1 마우스는
미엘린 희돌기아교세포 당단백질에 의한
실험적 자가면역성 뇌척수염(EAE)으로부터 보호되었으며,
이는 항체 결합 미엘린의 FcR 매개 흡수가
다발성 경화증의 발병에 관여한다는 것을 시사합니다 [33].
따라서
자가항체 매개 항원 흡수는
T세포 매개 자가면역 질환의 발병 기전에서 중요한 메커니즘일 수 있습니다.
자가면역의 병원성 기전으로서
자가항체 매개 항원 섭취에 대한 추가적인 지원은
형질전환 마우스가 흉선과 췌장 베타 세포 모두에서
오발부민(OVA)을 "자기"로 발현한 Harbers 등의 우아한 연구에서 비롯됩니다 [34].
수지상 세포에 의한
수지상 세포의 당뇨병 유발 CD8+ OVA 반응성 T 세포에 대한 OVA의 발현은
OVA에 특이적인 항체의 투여에 의해 현저하게 자극되었습니다.
이러한 반응은
활성화 FcγR이 없는 마우스에서는 관찰되지 않았으며,
이는 항체 기반 이펙터 T 세포 활성화가 실제로 FcγR에 의존한다는 것을 나타냅니다.
그러나
자가 항체가 항상 생체에 해를 끼치는 것은 아니지만
자연적인 자가 항체는 건강한 사람에게서 흔히 발견됩니다.
이러한 항체의 대부분은
IgM 동형이며 보호 기능이 있는 것으로 추측되고 있습니다.
이러한 기능 중 하나는
죽어가는 세포와 노화된 세포의 제거이며,
생쥐에서 자연 IgM 자가항체는
세포 사멸 세포에 특이적으로 발현되는 에피토프에 결합하여[37, 38]
이러한 세포의 제거를 강화하여 병원성 자가면역 반응을 유발할 수 있습니다[39, 40].
분비된 IgM의 부족은
세포사멸 세포의 제거 부족으로 인한 병
원성 IgG 자가항체 및 자가면역 질환의 증가와 상관관계가 있는 것으로 나타났습니다[41-43].
외부 항원에 노출되지 않고 발생하는
생쥐의 자연 자가항체는
B1 세포라고 하는 B 세포의 하위 집합에서 분비되며 [44, 45],
최근 인간에서도
유사한 B 세포 하위 집합이 확인되었습니다 [46].
SLE 환자의 경우,
세포 사멸과 관련된 높은 수준의 IgM은
건강한 쌍둥이 SLE 환자는
종종 이러한 자가항체 수치가 더 높습니다[49].
자연 자가항체에 의한 또 다른 보호 메커니즘은
병원성 자가항체가 자가항원과 반응하는 것을 차단하는 것이며[50],
dsDNA에 특이적인 자연 IgM의 역가는
SLE에서 사구체신염(GN)의 중증도와 반비례합니다[51, 52].
B세포가 고갈된 마우스는
CD4+ 및 CD8+ T 세포의 수가 급격히 감소하고
기억 CD8+ T 세포가 현저하게 억제되므로
활성화된 B세포는
항체 생산 외에도 T 세포 기능을 조정하는 데 필수적입니다 [53, 54].
아래에서 설명하는 것처럼
B 세포가 T 세포 및 기타 면역 세포에 영향을 미칠 수 있는
몇 가지 항체 독립적인 메커니즘이 있습니다.
2.2. B Cells as Antigen-Presenting Cells
Especially at low antigen concentrations B cells function as superior APCs [55]. Other APCs (macrophages and dendritic cells) internalize antigen through pinocytosis, while B cells capture antigen through their antigen-specific B-cell receptors (BCRs) (Figure 1(c)). The ability of antigen-specific B cells to serve as efficient APCs has been demonstrated in several in vivo studies [56]. This mechanism is 1,000–10,000-fold more efficient than pinocytosis, and antigens can be successfully presented at very low concentrations, as those present in autoimmune diseases [57–59]. Moreover, the BCR-conferred antigen-specificity enables the B cells to focus the immune response to a specific antigen [60].
B cells serve as APCs in autoimmune diseases including rheumatoid arthritis and type 1 diabetes [61, 62]. Immunoglobulin-deficient mice in a model of autoimmune arthritis (proteoglycan-induced arthritis) did not develop arthritis. The observation that T cells isolated from proteoglycan-immunized transgenic mice that express membrane Ig (mIgM), but lack circulating antibodies, were unable to transfer disease suggested that these T cells were not adequately primed and that antigen-specific B cells may be required for this process. This was confirmed when direct targeting of proteoglycan to the BCR induced T cells competent to transfer arthritis [61].
The role of B cells as APC in type 1 diabetes is discussed in a separate chapter below.
특히
낮은 항원 농도에서
B 세포는 우수한 APC로 기능합니다 [55].
다른 APC(대식세포 및 수지상 세포)는
피노세포화를 통해 항원을 내재화하는 반면,
B세포는 항원 특이적 B세포 수용체(BCR)를 통해
항원을 포획합니다(그림 1(c)).
항원 특이적 B 세포가 효율적인 APC 역할을 하는 능력은 여
러 생체 내 연구에서 입증되었습니다[56].
이 메커니즘은 핀세포증보다 1,000~10,000배 더 효율적이며
자가면역 질환에 존재하는 것과 같이
매우 낮은 농도에서도 항원을 성공적으로 제시할 수 있습니다 [57-59].
또한,
BCR에 부여된 항원 특이성은
B 세포가 특정 항원에 면역 반응을 집중할 수 있게 해줍니다 [60].
B세포는
류마티스 관절염과 제1형 당뇨병을 포함한
자가면역 관절염(프로테오글리칸 유발 관절염) 모델에서
면역글로불린이 결핍된 마우스는
관절염이 발병하지 않았습니다.
막 Ig(mIgM)를 발현하지만
순환 항체가 없는 프로테오글리칸 면역 형질전환 마우스에서 분리한
T 세포가 질병을 옮기지 못한다는 관찰은
이러한 T 세포가 적절하게 프라이밍되지 않았으며
이 과정에 항원 특이적 B 세포가 필요할 수 있음을 시사합니다.
이는 프로테오글리칸을
BCR에 직접 표적화하면
관절염을 옮길 수 있는 T세포가 유도되는 것으로 확인되었습니다[61].
제1형 당뇨병에서 APC로서 B 세포의 역할은 아래 별도의 장에서 설명합니다.
2.3. Proinflammatory Cytokine Secretion
Activated B cells can secrete proinflammatory cytokines like interleukin-6 (IL-6), interferon-gamma (IFN-γ), IL-4, and TGF-beta [63–65]. These inflammatory mediators modulate the migration of dendritic cells, activate macrophages, exert a regulatory role on T-cell functions, and provide feedback stimulatory signals for further B-cell activation.
활성화된 B 세포는
인터루킨-6(IL-6),
인터페론-감마(IFN-γ),
IL-4 및 TGF-베타[63-65]와 같은
염증성 사이토카인을 분비할 수 있습니다.
이러한 염증 매개체는
수지상 세포의 이동을 조절하고,
대식세포를 활성화하며,
T세포 기능에 조절 역할을 하고,
추가적인 B세포 활성화를 위한 피드백 자극 신호를 제공합니다.
2.4. Modulation of Antigen Processing and Presentation
Besides facilitating antigen uptake, both membrane-bound and soluble antibodies can modulate the processing pattern of the antigen [66–69] (Figure 1(d)). Depending on the antigenic epitope recognized by the antibody or the BCR of the B cell, different T-cell determinants are presented on the MHC molecule [67, 70–73]. Indeed proteolysis of antigen-antibody complexes yielded protein fragments that were not observed in the absence of antibody [74]. This might have consequences for the ensuing T-cell response, in particular when otherwise cryptic T-cell determinants are presented. This bias in processing of antigen complexed with antibody may stem from antibody-mediated protection of distinct peptide sequences from degradation and/or sequestering of peptide sequences and interference with the loading of peptides onto MHC molecules [75].
The relevance of this mechanism in autoimmune diseases was suggested by studies showing that antibodies to thyroglobulin could augment or suppress processing and presentation of pathogenic T-cell determinants [76] and will be discussed further in the T1D chapter.
막 결합 항체와 용해성 항체 모두 항원 흡수를 촉진하는 것 외에도
항원의 처리 패턴을 조절할 수 있습니다 [66-69](그림 1(d)).
항체에 의해 인식되는 항원 에피토프 또는
B세포의 BCR에 따라 MHC 분자에는
서로 다른 T세포 결정인자가 나타납니다[67, 70-73].
실제로 항원-항체 복합체의 단백질 분해는
항체가 없을 때는 관찰되지 않는
단백질 단편을 생성했습니다 [74].
이는 특히
비밀스러운 T세포 결정 인자가 제시될 때
후속 T세포 반응에 영향을 미칠 수 있습니다.
항체와 복합된 항원의 처리에서 이러한 편향성은
펩타이드 서열의 분해 및/또는 격리,
펩타이드가 MHC 분자에 로딩되는 것을 방해하는 항체 매개 보호에서 비롯될 수 있습니다 [75].
자가면역질환에서
이 메커니즘의 관련성은
티로글로불린 항체가
병원성 T세포 결정인자의 처리 및 제시를 증강하거나
억제할 수 있다는 연구 결과에 의해 제시되었으며[76],
T1D 장에서 자세히 논의할 예정입니다.
2.5. Ectopic Germinal Centers
B cells aid in the de novo generation of ectopic germinal centers (GCs) within inflamed tissues that can be observed during periods of chronic inflammation [77]. These ectopic structures are probably not a unique disease-specific occurrence, but a consequence of chronic inflammation. Activated T and B cells that infiltrate the site of chronic inflammation express membrane-bound lymphotoxin α 1 β 2 (LTα 1 β 2) [78]. High levels of LTα 1 β 2 eventually promote the differentiation of resident stromal cells into follicular dendritic cells (FDCs) and the development of ectopic GCs [79, 80]. These structures are similar to the GCs of secondary lymphoid organs and have been described in systemic lupus erythematosus, Hashimoto's thyroiditis, Graves' disease, rheumatoid arthritis, Sjögren's syndrome, multiple sclerosis, and type 1 diabetes [81–83]. The function and potential pathogenic role of ectopically formed lymphoid structures within inflamed tissues remains unclear. However, plasma cells residing within the ectopic GCs secrete autoantibodies [84], making it plausible that ectopic GCs have a role in the maintenance of immune pathology [85, 86].
Recent research has demonstrated that B cells are also involved in the inhibition of inflammatory immune responses, a function carried out by a subpopulation of B cells fittingly named regulatory B cells or Bregs.
3. IL-10 Secreting B Cells and Regulatory B Cells
A role of B cells in the inhibitory regulation of immune responses was initially suggested in autoimmune mice, where absence of B cells led to increased inflammation [87–89]. Transfer of wild-type B cells, but not IL10-negative B cells, reversed the inflammatory response [90], and IL-10 producing B cells were shown to suppress inflammation in mouse models of autoimmune diseases [91–93]. The significance of this anti-inflammatory cytokine was further supported by the finding that IL-10-deficient mice showed more severe disease accompanied with an increase in Th1 cytokine levels [88, 94, 95] and lower levels of regulatory T cells [96]. IL-10 is secreted by monocytes, Th2 T cells, regulatory T cells, and a rare subset of B cells. These IL-10 secreting B cells [97–100] can suppress CD4+ T cell responses and prevent autoimmune disease in mouse models and have been fittingly named regulatory B cells or Bregs [98–100]. The involvement of Bregs in human disease was first suggested by the observation that B-cell depletion can exacerbate Th-1-mediated autoimmune conditions such as ulcerative colitis [101] and psoriasis [102], and IL-10 producing B cells have been identified in humans [65]. For detailed discussions of Bregs please refer to other excellent reviews [99, 103].
4. B-Cell Tolerance
B-cell tolerance is established at multiple checkpoints throughout B-cell development, both in the bone marrow and the periphery. It has been estimated that 50% to 75% of newly produced human B cells are autoreactive and must be eliminated by tolerance mechanisms [104]. Induction of B-cell tolerance starts in the bone marrow. The major elimination mechanisms are receptor editing, clonal deletion, and anergy [105–107]. Defects in this early tolerance induction have been observed in subjects with rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes [53, 108–110].
Once autoreactive B cells are removed, the immature B cells leave the bone marrow and migrate to the spleen, where they may encounter autoantigen not present in the bone marrow. B cells with high avidity to autoantigen are deleted, while low-avidity or very-low avidity interactions lead to anergy or ignorance, respectively [111].
An encounter with true foreign antigen triggers the migration of the B cell to the T-cell zone of GCs, and activation by antigen-specific CD4+ T cells. During the ensuing rapid proliferation phase B cells undergo somatic hypermutation predominantly of the variable regions of their immunoglobulins. Only those B cells that express antibodies with increased affinity are selected to survive and exit the GC as antibody producing plasma cells or memory cells (for details see [112]).
B세포 내성은
골수와 말초 모두에서 B세포 발달 전반에 걸쳐
여러 체크포인트에서 확립됩니다.
새로 생성되는 인간 B 세포의 50~75%는
자가 반응성이며
내성 메커니즘에 의해 제거되어야 하는 것으로 추정됩니다[104].
B세포 내성 유도는
골수에서 시작됩니다.
주요 제거 메커니즘은
수용체 편집,
클론 결실 및 알레르기가 있습니다 [105-107].
이러한
초기 내성 유도의 결함은
류마티스 관절염,
전신성 홍반성 루푸스,
제1형 당뇨병 환자에서 관찰되었습니다[53, 108-110].
자가 반응성 B 세포가 제거되면
미성숙 B 세포는 골수를 떠나 비장으로 이동하여
골수에 존재하지 않는 자가 항원을 만날 수 있습니다.
자가 항원에 대한 선호도가 높은 B 세포는 삭제되고,
선호도가 낮거나 매우 낮은 상호 작용은
각각 알레르기 또는 무지로 이어집니다 [111].
진정한 외부 항원과의 만남은
B세포가 GC의 T세포 영역으로 이동하고
항원 특이적 CD4+ T세포에 의해 활성화됩니다.
이후 급속한 증식 단계에서
B 세포는 주로 면역 글로불린의 가변 영역에서
체세포 과돌연변이를 겪습니다.
증가된 친화력을 가진 항체를 발현하는 B 세포만이 살아남아
항체 생산 형질세포 또는
기억세포로 GC를 빠져나가도록 선택됩니다(자세한 내용은 [112] 참조).
4.1. Loss of Tolerance
Any of the above-discussed tolerance checkpoints can be faulted by genetic mutations allowing autoreactive B cells to survive. Some of these mutations have been identified in mouse models of autoimmune diseases with parallel findings in human disease.
Faulty negative selection at the immature B cells stage: NZM2410 mice spontaneously develop severe lupus nephritis at an early age. These mice carry the lupus susceptibility locus Sle1 containing at least three subloci, Sle1a, Sle1b, and Sle1c, involved in B-cell tolerance and activation of CD4+ T cells [113]. Using Sle1 congenic C57Bl6 mice, Kumar and colleagues [114] showed that mutations located within the Sle1 induced loss of B-cell tolerance through impaired negative selection of autoreactive B cells at the immature B-cell stage.
Increased B-cell signaling by overexpression of BCR signal-enhancing molecules or deficiency of molecules inhibiting BCR signaling: CD19 is a B-cell surface molecule that decreases the threshold for BCR stimulation. Hyperexpression of CD19 in mice led to increased levels of serum antibodies and increased B-cell activation, while the loss of CD19 reversed these phenotypes [115–119]. Deficiency of molecules that inhibit BCR-signaling, such as SHP-1 [120], Lyn [121], or FcγRIIB [122], causes increased B-cell signaling and initiates development of systemic autoimmunity in mice. The inhibitory FcγRIIB is expressed on B cells, where it regulates activating BCR signals. Lack of FcγRIIB expression leads to autoimmunity and autoimmune diseases [122–124]. The importance of FcγRIIB in human autoimmunity is exemplified by the finding that B cells from patients with lupus express lower levels of FcγRIIB on their surface due to polymorphisms in their FcγRIIB promoter [125], or the receptor itself [126, 127].
Generation of autoreactive immunoglobulins during somatic hypermutation: during affinity maturation the massive somatic hypermutations can also cause the inadvertent development of autoreactive immunoglobulins. While normally the resulting autoimmune B cells may either not receive necessary survival signals [128] or be eliminated, they accumulate in autoimmune diseases.
Increased survival of autoreactive B cells: B-cell activation factor (BAFF) is a B-cell survival factor and overexpression of BAFF in transgenic mice led to an expansion of peripheral B cells with higher autoantibody levels and the development of a lupus-like disease in the animals [28]. Elevated serum levels of BAFF have been found in patients with rheumatoid arthritis, systemic lupus erythematosus, and primary Sjörgren's syndrome [129–131]. These observations make BAFF a potential target for therapy [132, 133]. Indeed neutralization of BAFF was shown to be associated with loss of mature B cells [134] and reduced symptoms of autoimmune diseases in animal models [135, 136].
In the following the role of B cells in autoimmune diseases will be discussed in the context of systemic lupus erythematosus, rheumatoid arthritis, and type 1 diabetes. Systemic lupus erythematosus is a classic B-cell-mediated autoimmune disease, while rheumatoid arthritis and type 1 diabetes were initially considered to be predominantly T cell mediated. However recent studies suggest a role of B cells in the pathogenesis of these autoimmune diseases, as will be discussed in detail below.
Systemic Lupus Erythematosus (SLE) is a complex autoimmune disease, characterized by hyperglobulinemia, immune complex deposition, and end organ damage. B cells have been identified as major contributors to SLE, and B-cell depletion in SLE animal models abrogated the development of disease [54, 137]. Indeed, generalized B-cell hyperactivity has been documented in several murine models of lupus [138] and is also evident in patients with lupus [139, 140], where the number of B cells at all stages of activation is increased during active disease [141]. Both the decrease in proapoptotic genes and the increase in prosurvival gene expression have been suggested to cause this prolonged half-life of B cells in SLE (see also above).
A pathogenic role of autoantibodies in SLE is supported by the observation that the passive transfer of anti-DNA antibodies induces distinct features of lupus nephritis in healthy animals [142, 143]. Autoantibodies in SLE contribute to end organ damage in glomerulonephritis (glomerular antibodies and anti-DNA antibodies) [144–146], congenital heart block (anti-Ro antibodies) [147], and thrombosis (anticardiolipin antibodies) [148]. Other autoantibodies are directed to diverse self-molecules, most notably antinuclear antibodies directed to double stranded DNA (dsDNA) [149], and small nuclear ribonucleoprotein (snRNP). However, B cells also have antibody-independent effects on the SLE pathogenesis. These functions include antigen presentation, costimulation of T cells, and secretion of proinflammatory cytokines. This role was evaluated in a set of experiments conducted by Chan and colleagues, where B cells in a SLE mouse model carried a mutation that prevented the secretion of antibodies [54]. Thus these animals had B cells but were devoid of circulating antibodies. Despite the absence of autoantibodies, the mice developed nephritis, indicating an antibody-independent effect of B cells. B-cell-deficient MRL/lpr mice remain disease-free and fail to develop activated CD8+ and CD4+ T cells found in B-cell-sufficient mice, a finding attributed to loss of B cell-CD4 T cell interactions [150].
The dual effect of IL-10 as a B-cell stimulator and inhibitor of T-cell activation is exemplified in SLE [151]. In mice models for SLE, IL-10 appears to exert mainly its above-discussed anti-inflammatory effect and IL-10-deficient mice develop a more severe disease with increased proinflammatory cytokine levels [152], while transfer of IL-10 producing B cells induced the expansion of regulatory T cells [96]. However, in human SLE IL-10 promotes disease, IL-10 serum levels are significantly elevated and correlate with disease activity [153] and IL-10 induced a significant increase of anti-DNA antibody secretion in cultured PBMCs from SLE patients [154]. This antibody secretion was significantly reduced in the presence of neutralizing IL-10-specific antibodies [155] and treatment with IL-10-specific monoclonal antibodies led to marked improvement in participants of a small clinical trial [156]. The protective effect of IL-10 in mice appears to be mediated through T-cell regulation, as IL-10 overexpression in a mouse model for lupus resulted in reduced T-cell activation, while B-cell phenotypes remained unaffected [151]. In SLE patients immune cells that normally suppress B-cell activation are defective and do not counteract the IL-10-mediated stimulation of B cells resulting in the subsequent secretion of autoantibodies [157].
Rheumatoid Arthritis (RA) is a chronic inflammation of the joint capsule (synovium) and synovial membranes, associated with proliferation of synovial fibroblasts and macrophages, leading eventually to cartilage injury and bone erosion [158]. While T cells are a major component in the pathogenesis, several observations suggest that B cells are necessary for the development of the disease, as B-cell deficiency in RA animal models abrogates disease [159, 160], and autoimmune T cells alone are not sufficient to induce disease [161]. At least two mechanisms of B-cell involvement are currently considered: the production of autoantibodies and antigen presentation. Autoantibodies in patients with RA typically target several autoantigens, including rheumatoid factor (RF), type II collagen (CII), and citrullinated proteins (ACPA). A model for the pathological role of RA-associated autoantibodies will be discussed for autoantibodies directed to CII. These autoantibodies are found in ~70% of patients with early RA [162–164] both in their serum and synovial fluids. A pathogenic role of CII-specific antibodies was indicated in an animal model termed collagen-induced arthritis (CIA), where immunization of animals with CII induced the development of CII antibodies [165] and triggered arthritic symptoms [166–168]. Moreover, arthritic symptoms were also observed after passive transfer of CII-reactive serum obtained from CIA animals [169], patients with RA [170], or monoclonal antibodies specific to CII [165, 171] to healthy recipient animals, further supporting a pathological role of CII antibodies. CII autoantibodies are thought to mediate the formation of immune complexes in the joint, followed by complement activation and inflammatory cell recruitment. After FcγR ligation, the activated cells secrete proinflammatory cytokines, further activating an immune reaction consisting of synovial macrophages and infiltrating mononuclear cells with the eventual release of tissue-degrading enzymes that cause cartilage damage [172]. CII autoantibodies may also have a direct pathogenic function, which occurs in the absence of inflammatory mediators [173]. Here the antibodies modify the synthesis of collagen fibrils effecting cartilage synthesis and stability [174–176], possibly through steric hindrance of collagen epitopes that are important for the formation of collagen fibrils [177–179].
Type 1 Diabetes (T1D) is an organ specific autoimmune disease, characterized by the destruction of the insulin-producing beta cells in the pancreas. During progression towards T1D the pancreatic islets are infiltrated by mononuclear cells consisting of CD4+ and CD8+ T cells, B cells, macrophages, and dendritic cells [180, 181]. Both CD4+ and CD8+ T cells contribute to the ultimate attack on the beta cells [182], but in recent years the pathogenic role of B cells is beginning to emerge [183, 184]. A major hallmark of the autoimmunity leading to T1D is the presence of autoantibodies to beta cell antigens. At the time of clinical diagnosis more than 90% of patients present at least one of the T1D-associated autoantibodies [185]. The four beta cell antigens most frequently targeted by autoantibodies are insulin [186], the smaller isoform of glutamate decarboxylase (GAD65) [187], protein-tyrosine-phosphatase-like protein IA-2 [188], and the zinc transporter 8 (ZnT8) [189]. These autoantigens are also targeted by autoreactive T cells, suggesting a collaborative interaction between T and B cells [190]. No direct pathogenic role has been assigned to these autoantibodies and they are generally viewed as markers only. However a potential role of GAD65Ab in enhanced antigen uptake has been suggested [191]. Stimulation of GAD65-specific T-cell clones with human recombinant GAD65 was tested in the presence of sera obtained from GAD65Ab-positive T1D patients and GAD65Ab-negative T1D patients. Only sera from GAD65Ab-positive patients significantly enhanced T-cell stimulation. Moreover, this effect was inhibited by monoclonal antibodies to the FcR, suggesting Fc-mediated uptake of GAD65 complexed with GAD65Ab as the underlying mechanism.
However, the major mechanism by which B cells contribute to T1D development is the antibody-independent presentation of beta cell antigens [190, 192, 193]. Nonobese diabetic (NOD) mice deficient of mature B cells do not develop T1D [193–199]. In the absence of B cells, NOD mice showed significantly lower numbers of CD4+ and CD8+ T cells in their insulitic lesions [62, 195, 198–200], suggesting a role of B cells in the activation of autoreactive T cells. The function of B cells as APCs was illustrated in NOD mice whose B cells were rendered MHC class II deficient [201]. Although these animals retained their ability to present antigen via dendritic cells and macrophages, they were protected from diabetes development. However, the presence of insulitis in B-cell-deficient mice [62] and the report of at least one B-cell-deficient T1D patient [202] indicate that B cells may not be absolutely essential for the development of T1D and can be substituted by other APCs. As discussed above, B cell can focus the immune response towards a specific antigen. NOD mice that expressed only B cells specific to an irrelevant antigen (Hen Egg Lysosome) did not develop an autoantigen-specific T-cell response and remained healthy, indicating that only autoantigen-specific B cells enhance the development of T1D in the NOD mouse [203]. We will discuss the role of autoantigen-specific B cells exemplified by GAD65-specific B cells. Although GAD65 levels in murine pancreatic beta cells are very low, it is a major autoantigen in the pathogenesis of T1D in the NOD mouse [204]. GAD65-specific T cells have been demonstrated in both T1D patients and the NOD mouse [205–209]. Adoptive transfer of GAD65-reactive T cells isolated from NOD mice caused recipient animals to develop T1D [207, 210], supporting the concept of diabetogenic GAD65-specific T cells in the pathogenesis of T1D. Importantly, the development of these GAD65-specific T cells depends on the presence of B cells [190, 192, 203]. The finding that reconstitution of B-cell-depleted NOD mice with B cells reinstated T1D only if the repopulating B cells were primed with GAD65 [190] suggests that B-cell-mediated presentation of GAD65 stimulates GAD65-reactive T effector cells to target pancreatic beta cells. It is however not only the antigen specificity, but also the epitope specificity of the B cells that affects the T-cell response. GAD65-specific B-cell hybridomas with different epitope specificities were tested for their capacity to stimulate GAD65-specific T-cell clones. Those T-cell clones whose epitope lays outside of the BCR epitope showed increased T-cell responses, while T-cell clones whose epitope lays inside the BCR epitope showed suppressed responses, suggesting that the BCR epitope specificity can promote the presentation of some T-cell determinants, while suppressing that of others [211, 212].
Based on the promising results of B-cell depletion in the prevention of T1D in NOD mice, the effect of B-cell depletion on human T1D was tested in a phase II multicenter clinical trial on newly diagnosed human T1D patients [213]. One year after treatment a delay in the loss of beta cell function as shown by the preservation of C-peptide was demonstrated. Moreover, patients required less insulin and had better overall blood glucose control. These results confirm that B cells contribute also to human T1D.
Gathering the current understanding of B cells in T1D, the following mechanisms have been suggested (Figure 2). Beta cell antigen is taken up via BCR by antigen-specific B cells (1) and presented on MHC class II molecules to CD4+ T cells (2). Activated CD4+ T cells provide help to B cells (3). B cells differentiate to plasma cells and secrete autoantibodies (4). These autoantibodies form autoantigen-autoantibody complexes that bind to the FcγR on other APCs (5). This enhanced antigen presentation eventually triggers both natural killer cells and CD8+ T cells to attack the pancreatic beta cell.
Model of pathogenic function of B cells in type 1 diabetes. Islet cell antigen released from the pancreatic beta cells is being taken up at low antigen concentrations by antigen-specific B cells, which present the antigen determinants to CD4+ T cells. T cells provide help to the B cells to eventually differentiate into antibody secreting plasma cells. Autoantibodies can now bind to the autoantigen and the resulting autoantibody/autoantigen complexes are efficiently taken up via FcR present on other APCs. This enhanced autoantigen uptake and presentation finally activates cytotoxic CD8+ T cells, which carry out the killing of the beta cells.
5. B-Cell Depletion
The growing understanding that B cells play a pathological role also in autoimmune diseases that are traditionally viewed as T cell mediated led to B-cell depletion treatment not only in diseases that are clearly B cell dominated, but also in autoimmune diseases that are traditionally viewed as T cell mediated, such as T1D.
B-cell depletion can target a number of different B-cell molecules, either with the goal of B-cell elimination, or the suppression of survival. Four major classes of B-cell targeting drugs have been evaluated for the treatment of autoimmune diseases: neutralization of survival factors BAFF and APRIL [214], killing of B cells using monoclonal antibodies directed to CD19, CD20, and CD22 [215–217], induction of apoptosis using reagents targeting the BCR itself or BCR associated transmembrane signaling proteins such as CD79 [193, 218], and ablation of the formation of ectopic GCs by antibodies against lymphotoxin-β receptor (LTβR) [219].
B-cell depletion for treatment of human autoimmune diseases is often accomplished through antibodies targeting the surface molecule CD20 (e.g., Rituximab and Ofatumumab). Treatment with these antibodies depletes B cells by a combination of antibody-mediated cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and antibody-triggered apoptosis [220] (Figure 3). The CD20 density on B cells appears to be important for CDC, since it is highly correlated with CDC [221]. CD20mAb/CD20 immune complexes aggregate in microdomains, where the antibodies' Fc regions are bound by C1q, leading to complement activation [222]. CD20 may also act as a signaling molecule to trigger apoptosis when engaged with CD20mAb [223, 224].
B-cell depletion with CD20 (Rituximab). Anti-CD20 mAb can direct the killing of B cells by antibody-dependent cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), or apoptosis. ADCC is triggered by the interaction between the Fc region of the antibody and the FcR on effector cells of the immune system. In CDC the Fc region is bound by the complement component C1q, which triggers a proteolytic cascade. Apoptosis occurs when CD20 molecules are cross-linked by anti-CD20 mAb in lipid rafts and activate signaling pathways leading to cell death.
B-cell depletion using Rituximab has been used for the treatment of a number of autoimmune and chronic inflammatory diseases [213, 225, 226]. Rituximab treatment results in nearly undetectable circulating B-cell levels one month after therapy and B cell counts remain low for 6–12 months [227]. Because the drug targets B cells expressing surface CD20, mature and memory CD20+CD27+ B cells in blood and primary lymphoid organs are effectively depleted, while long-lived plasma cells are not directly depleted [228], and Rituximab treatment appears not to affect circulating IgG levels [229], while reducing circulating IgM levels [230]. This effect of Rituximab is illustrated by the observation that immunization within the first 9 months after Rituximab treatment results in significantly reduced antibody responses, which develop from IgM-positive B cells [231, 232]. It is therefore of interest that for some autoimmune diseases B-cell depletion was reported to be associated with a decrease in IgG autoantibody titers [77] and specific depletion of autoreactive B cells by CD20mAb was demonstrated in mice [233]. As bone marrow stem cells and early B-cell precursors (pro-B cells) do not express CD20 [234], the new naïve B cells repopulate the B-cell compartment once the drug has cleared the system, allowing the immune response to return to normal. Disease relapses in about 50% of patients either at the time that B-cell numbers increase to pretreatment levels or within 3 months, while in other cases clinical relapse can be delayed for years [235]. Additional Rituximab courses can induce subsequent remission [236]. Multiple Rituximab courses are often associated with progressive decrease in circulating IgM [237] and IgG levels [238].
The antibody-independent effect of Rituximab treatment may be due to the elimination of B cells as APC and subsequent reduced stimulation of T cells [239, 240]. However, not all CD20+ B cells are equally affected by Rituximab treatment. B cells located in the peritoneal cavity are surprisingly resistant to depletion [241]. While these B cells express normal CD20 densities and are bound by CD20mAb, only about 50% of these cells are depleted. These location-dependent sensitivities to CD20mAb-mediated depletion could have significant consequences for therapy and may be the reason of the heterogeneity of results in human clinical trials. Other factors such as gender, age, and weight [242] and immunological profile [243] affect the outcome of Rituximab treatment. The major side effect of B-cell depletion is the risk of severe infections, which needs to be taken into consideration when evaluating the risks and benefits of B-cell depletion [244, 245].
In summary, B-cell depletion offers a promising therapy for the treatment of a variety of autoimmune diseases. The treatment is usually well tolerated; however, adverse events include infusion reactions, infections, and hypogammaglobulinemia.
6. Conclusions and Future Directions
The traditional concept of T-cell-mediated and autoantibody-mediated autoimmune diseases needs to be adjusted to reflect the interaction of different immune cells in autoimmune pathogenesis. The recognition of the contribution of B cells in the pathogenesis of autoimmune diseases, which are traditionally viewed as T cell mediated, led to promising immune-modulating therapies.
Global B-cell depletion eliminates both protective and pathogenic B cells. The success of B-cell depletion is therefore dictated by the extent of depletion of protective versus pathogenic B cells. The hopes that B-cell depletion would allow the restoration of immunological tolerance with long-term remission were not fulfilled, as is evident from the recurrence of autoimmune disease after the B-cell compartment is replenished. Selective depletion of antigen-specific B cells may provide an alternative to global B-cell depletion. This approach has the additional advantage that unlike Rituximab treatment it may also eliminate CD20-long-lived autoreactive plasma cells.
Several mechanisms are currently investigated in different in vitro and in vivo models of autoimmune diseases, a few of which will be discussed here.
Autoantigens can be fused to the IgG1 Fc domain to activate complement and FcR-dependent effector cell responses. This approach has been successfully evaluated in vitro and in vivo for the treatment of multiple sclerosis by autoantigen fused to Fc, which induced the effective and specific effector lysis of autoantigen-specific B cells [246]. An inhibitory B-cell signal can be induced by cross-linking of the autoantigen-specific BCR with the inhibitory FcγRIIb. Autoantigen fused to an FcγRIIb-binding mAb successfully reduced autoantibody levels and disease symptoms in lupus-prone MRL/lpr mice [247–249]. Autoantigen can also be coupled to an antibody specific to complement receptor 1 (CR1). CR1 negatively regulates the proliferation and differentiation of activated B cells after binding C3b [250]. In a small clinical trial SLE patients treated with dsDNA coupled to a CR1-specific monoclonal antibody showed a significant reduction of dsDNA autoantibody titers [251]. In an early study, Blank et al. employed anti-idiotypic antibodies directed to a pathogenic anti-DNA idiotype. Administration of this anti-idiotypic antibody alone or coupled to the cytotoxin saporin induced a significant reduction in anti-DNA antibody titer and diminished clinical manifestation in lupus-prone mice [252]. In a similar approach we demonstrated that GAD65Ab-specific anti-idiotypic antibodies protected NOD mice from development of T1D [253]. In addition to the direct elimination of antigen-specific B cells, autoantigen-fusion proteins can also bind pathogenic autoantibodies and route them to clearance.
Recently Bollmann proposed the targeted elimination of autoantigen-specific B cells using artificial antigens linked to magnetic nanoparticles. Here the autoantigen-specific B cells would be removed in an extracorporeal filtration method in an attempt to suppress or cure the autoimmune response [254].
The feasibility of these specific B-cell depletion approaches needs to be further evaluated; however, they offer new therapeutic options for the treatment of autoimmune diseases.
Acknowledgments
This work was supported by the National Institutes of Health (DK26190) and the Juvenile Diabetes Research Foundation.
Abbreviations
| AChR: | ACh receptor |
| ACPA: | Citrullinated proteins |
| ADCC: | Antibody-dependent-cell-mediated cytotoxicity |
| APCs: | Antigen presenting cells |
| BAFF: | B-cell activation factor |
| BCR: | B-cell receptors |
| Bregs: | Regulatory B cells |
| CII: | Type II collagen |
| CDC: | Complement-dependent cytotoxicity |
| FcR: | Fc receptor |
| FcγR: | Fc gamma receptor |
| FDCs: | Follicular dendritic cells |
| GAD65: | 65 kD isoform of glutamate decarboxylase |
| GC: | Germinal centers |
| IA-2: | Protein-tyrosine-phosphatase-like protein |
| IFN-γ: | Interferon-gamma |
| LTα1β2: | Membrane-bound lymphotoxin α 1 β 2 |
| LTβR: | Lymphotoxin-β receptor |
| MHC: | Major histocompatibility complex |
| mIgM: | Membrane IgM |
| NOD: | Nonobese diabetic |
| OVA: | Ovalbumin |
| RA: | Rheumatoid arthritis |
| RF: | Rheumatoid factor |
| SLE: | Systemic lupus erythematosus |
| T1D: | Type 1 diabetes |
| TPO: | Thyroid peroxidase |
| TSH: | Thyroid stimulating hormone |
| ZnT8: | Zinc transporter 8. |
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