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B cells, splenic marginal zone, and complement -as opsonin and inflammatory mediator- in the immunopathogenesis of virally induced Type 1 Diabetes

Final Activity Report Summary - CODIA (B cells, splenic marginal zone, and complement -as opsonin and inflammatory mediator- in the immunopathogenesis of virally induced Type 1 Diabetes)

This Marie Curie project was designed to investigate links between complement -as modifier of the adaptive immune response, and important part of the innate immune and inflammatory responses-, and the onset and pathogenesis of autoimmune Type 1 Diabetes (T1D). The project was carried out sequentially at two "Host Institutions", starting out at the Institute of Experimental Immunology at the University of Zurich (Switzerland) until the management transition there resulted in a halt of research per February 29th, 2008. From then on, the project was successfully transferred to the Institute for Medical Microbiology, Immunology and Hygiene, at the Technical University of Munich (Germany).

The aim of identifying roles for the complement system in T1D development was approached by means of two independent mouse models for T1D: 1) the non-obese diabetic mouse (NOD), which owes its spontaneous development of diabetes to its complex genetic composition, and 2) transgenic RIP-LCMV mouse models, which rely on a specific immunogenic stimulus, such as infection, to induce autoimmune diabetes.

T1D development in NOD mice was shown to be muted by blockade of complement receptors (CR) 1 and 2 (Noorchashm, Moore et al. 1999).These receptors are expressed at high levels on a subset of B cells, Marginal Zone (MZ) B cells. CR1/2 not only aid binding of antigenic material via complement C3 activation-products, but serve as immune response-enhancing co-receptors for the antigen-specific B cell receptor as well. We found that B cells infiltrate into follicular areas of the spleen that are normally reserved for T cells. As this uncommon breach in separation can be observed most clearly around the time of diabetes development, we propose that this blurring of B and T cell areas may well facilitate productive contacts between B cells and autoreactive T cells. Indeed, an important hypothesis at the outset of this project, namely that specifically MZ B cells may be effective inducers of T1D in NOD mice, was confirmed in the literature during this project (Marino, Batten et al. 2008).

To identify through what mechanism the complement system may influence the development of autoimmune diabetes, a second aim was to isolate the role of CR1/2 in T1D from the complex genetic NOD background. We achieved this goal by including CR1/2 deficiency into a broad panel of complement factor deficient mice, crossed onto the RIP-LCMV transgenic T1D model. This model circumvents the genetic predisposition of the NOD mouse, and instead relies on a targeted immunogenic stimulus, typically in the form of low-level infection, to induce T1D. After evaluating various infectious regimens, we realised that uncovering the mechanism of complement in T1D required a more controllable immune stimulation than commonly used. We therefore involved ourselves in the development and testing of a replication deficient form of LCMV (rLCMV). Using this highly controllable mode of infection, we were able to identify specific complement factors as important players in the development of T1D in the RIP-LCMV mouse model of autoimmune diabetes.

Combined, our observations revealed a fundamentally different role for complement in T1D development in a genetically susceptible, versus an infection-induced model of autoimmune diabetes. Our results in the NOD model point toward an important role for B cells: principally capable of complement aided antigen-capture, -processing, and -presentation to T cells, B cells breach the normally strict follicular boundary separating the two lymphocyte lineages while T1D is developing. Moreover, while complement clearly plays a role in T1D in the RIP-LCMV model, this is likely due to pathways involving anaphylatoxin C5a, rather than CR1/2. C5a has been shown to influence T cell responses in a variety of ways, and we focus our current and future experiments on analysing complement´s influence on T1D-associated T cell responses in additional detail.

The project is continuing at the Institute for Medical Microbiology, Immunology and Hygiene in Munich.