Durable humoral immunity regulated by intrinsic CD28 function in plasma cells

描述（由申请方提供）：持续的长期抗体水平是保护性免疫的基石，但持久维持此类滴度的机制仍不清楚。流行的范例是通过在骨髓（BM）中有限数量的特定小生境内存活的长寿命浆细胞（LLPC）的子集的连续抗原非依赖性抗体产生而发生。然而，该范例的中心原则，即BM LLPC是由本质上不同的生物学定义的单独子集，并且它们实际上有助于长期抗体滴度，尚未得到直接证明。我们最近报道（Rozanski等人，JEM，2011年7月），长期体液免疫依赖于CD 28的浆细胞内在功能（最好表征为原型T细胞共刺激受体），其选择性支持BM LLPC的存活，而不是脾短寿命PC（SLPC）的存活。尽管LLPC和SLPC均表达CD 28，但CD 28信号传导和增强的存活仅发生在LLPC中（并且与T细胞不同，不需要外源性信号1）。在体内，即使有足够的T细胞帮助，浆细胞中CD 28的损失（或CD 28配体CD 80或CD 86的整体损失）也会导致LLPC群体的显著损失，BM中LLPC存活的半衰期从426天减少到63天，并且无法长期维持抗体滴度-对SLPC群体没有任何影响。在微环境/生态位方面，我们发现CD 80+树突状细胞（DC）与BM内的LLPC直接接触，髓样DC在体外以CD 28依赖性方式支持LLPC的长期存活，并且通过CD 80和CD 86连接的“反向信号”诱导DC产生前PC存活细胞因子IL-6，其对于维持体外免疫球蛋白（IG）产生是出乎意料地必需的。总之，这些发现确立了维持抗体滴度所必需的不同BM LLPC亚群的存在，并揭示了PC内在CD 28功能在浆细胞寿命和体液免疫中的先前未被认识但重要的作用。虽然先前的研究已经明确定义了CD 28在保护性和致病性（例如自身免疫）抗体应答中的作用，但这几乎完全归因于辅助性T细胞活化的共刺激。对这些文献的重新思考提出了一个重要的问题，即在T细胞和浆细胞中，有多少体液应答是由于CD 28的功能。因此，我们相信PC固有的CD 28功能在浆细胞生物学和持久体液免疫的产生中发挥着核心作用，但在这种情况下几乎完全没有特征。该提案的总体目标是确定CD 28活化调节PC存活、功能和体液免疫应答发展的具体机制。本提案的目的是：1）。确定调节PC存活和功能的CD 28活化下游的信号转导途径和分子靶点，2）。表征⑶ 80/⑶ 86 + LLPC BM基质生态位，和3）。在T或B细胞谱系中CD 28条件性敲除的小鼠中，表征PC与T细胞固有CD 28功能在体液免疫应答中的作用。

DESCRIPTION (provided by applicant): Sustained long-term antibody levels are the cornerstone of protective immunity, yet the mechanism(s) by which such titers are durably maintained remains unclear. The prevailing paradigm is this occurs via continuous antigen-independent antibody production by a subset of long-lived plasma cells (LLPC) that survive within a limited number of specific niches in the bone marrow (BM). The paradigm's central tenets however, that BM LLPC are a separate subset defined by intrinsically distinct biology and that they in fact contribute to long-term antibody titers, have not been directly demonstrated. We have recently reported (Rozanski et al, JEM July, 2011) that long-term humoral immunity is dependent on plasma cell-intrinsic function of CD28 (best characterized as the prototypic T cell costimulatory receptor), which selectively supports the survival of BM LLPC but not splenic short-lived PC (SLPC). Although LLPC and SLPC both expressed CD28, CD28 signaling and enhanced survival only occurred in the LLPC (and unlike T cells did not require an exogenous signal 1). In vivo, even with sufficient T cell help the loss of CD28 in the plasma cells (or the global loss of the CD28 ligands CD80 or CD86) caused significant loss of the LLPC population, reduction of the half-life of LLPC survival in the BM from 426 to 63 days, and inability to maintain antibody titers long-term - without having any effect on SLPC populations. On the microenvironment/niche side, we found that CD80+ dendritic cells (DC) were in direct contact with LLPC within the BM, that myeloid DC supported long-term LLPC survival in a CD28-dependent fashion in vitro, and that "backsignaling" via CD80 and CD86 ligation induced the DC to produce the pro-PC survival cytokine IL-6 that was unexpectedly essential for sustaining immunoglobulin (Ig) production in vitro. Altogether, these findings establish the existence of the distinct BM LLPC subset that is necessary to sustain antibody titers, and uncover a previously unrecognized but essential role for PC-intrinsic CD28 function in the longevity of plasma cells and humoral immunity. While previous studies have clearly defined a role for CD28 in both protective and pathogenic (e.g. autoimmune) antibody responses, this has been almost entirely attributed to costimulation of helper T cell activation. Reconsideration of this literature raises he important question of how much of the humoral response is due to CD28 function in T cells vs. plasma cells. Thus we believe that PC-intrinsic CD28 function plays a central role in plasma cell biology and the generation of durable humoral immunity, but has been almost entirely uncharacterized in this context. The overall goal of this proposal is to define the specific mechanisms by which CD28 activation regulates PC survival, function, and the development of humoral immune responses. The Aims of this proposal are to: 1). Define the signal transduction pathways and molecular targets downstream of CD28 activation that modulate PC survival and function, 2). Characterize the CD80/CD86+ LLPC BM stromal niche, and 3). Characterize the role of PC vs. T cell intrinsic CD28 function in humoral immune responses in mice conditionally knocked out for CD28 in the T or B cell lineage.