Memory immune responses are critical for long-lasting vaccine and natural immunity. Nonetheless, the identity, selection and maintenance requirements of memory B-cells remain poorly understood. Germinal centers (GC's) are sites at which memory B-cells develop. Somatic mutation and selection on mutants occur in the GC, and it is microenvironment that arises in B-cell follicles of secondary lymphoid organs after immunization (and ectopically in sites of chronic inflammation). It is comprised mainly of activated B-cells, along with some activated T-cells. A well developed GC is further subdivided into light and dark zones and is surrounded by a marginal zone. Also uniquely present in the GC are follicular dendritic cells (FDC's). These cells are not phagocytic and probably not bone marrow-derived; they are concentrated in the light zone, have Fc receptors, complement receptor 1 (CR1) and CD23. FDC's associate closely with B-cells and may be involved in B-cell selection and development, perhaps through their ability to retain immune-complexed Ag via FcR and CR1. However, their precise role has not been defined. When Gray and Skarvall presented data suggesting that persistent antigen is required in order to prevent the decay of B memory cells, it was proposed that Ag-Ab complexes retained on FDC's play a critical role in memory B-cell formation and maintenance. This view has gained currency, and at present, it is hypothesized that Ag deposition/retention on FDC is essential for: 1) the production and selection of memory B-cells; and 2) the maintenance of memory populations. However, these ideas have not been tested experimentally. To test these two hypotheses we have developed a novel Ig transgenic mouse which has B-cells but lacks circulating Ab. In this mouse, Ag-Ab complexes and Ag deposition on FDC's are eliminated, yet B-cell responses can still be monitored. In the preliminary experiments, the investigators discovered that robust primary immune responses occur in the absence of soluble Ab.
Specific Aims 1 and 2 propose to test the development, maturation and maintenance of memory B-cells in these mice, thereby defining the role of Ab-mediated Ag-deposition on FDC's in B-cell memory. The fact that normal primary responses occur in the absence of soluble Ab seems to violate a proposed requirement (based on the phenotype of CR1 knockout mice) for complement fixation by soluble Ab for optimal immune responses. These studies propose instead that membrane IgM naturally serves this function, and have devised a system to test this idea in Aim 3.
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