The second role of complement receptors CD21 and CD35 in regulating humoral responses is that they permit FDCs to trap antigen (Fig.1). FDCs concentrate in regions of ongoing immune responses, such as germinal centers, and they appear necessary for antibody responses. Germinal centers (see earlier section) promote somatic hypermutation within Ig heavy- and light-chain genes along with production of memory B lymphocytes and plasma cells. They can be divided into two regions, dark zone and light zone. To gain entry into the dark zone, B lymphocytes are activated by receiving above threshold signals from the CD21/CD19/CD81 co-receptor complex and BCR in combination with co-stimulation from helper T lymphocytes. Within the dark zone, activated B lymphocytes divide and mutate their Ig receptor genes. After several rounds of proliferation in the dark zone, B lymphocytes enter the light zone, where they are subjected to selection on antigen deposited on FDCs (i.e., clonal selection). The selection of high-affinity B lymphocyte clones into memory B-lymphocyte and plasma cell pools ensures future protection against repeat antigen exposure.
Fig1. ROLE OF COMPLEMENT-TAGGED ANTIGEN IN DIRECTING B-LYMPHOCYTE ACTIVATION AND FORMATION OF MEMORY B LYMPHOCYTES. Mature B lymphocytes survey secondary lymphoid tissues in search of antigen. Survival of mature B lymphocytes after antigen contact and T-cell help within splenic follicles depends on coreceptor signals through CD21/CD35. Lymphocytes receiving requisite signals expand and continue to differentiate within germinal centers, where CD21/CD35 is again important. B lymphocytes not receiving complement–ligand interactions in germinal centers die. In addition, complement-mediated deposition may localize antigen to follicular dendritic cells (FDCs), thereby providing the substrate for B-lymphocyte selection. Selection and differentiation in germinal centers lead to production of long-lived memory B lymphocytes and effector cells. The lifespan of memory B lymphocytes may also depend on continued interaction of antigen deposited on FDCs with CD21/CD35 in the spleen and in bone marrow. IgG, Immunoglobulin G.
How antigen is retained on FDCs, both for primary B-lymphocyte responses and for long-term memory responses, is subject to intense research. However, supporting evidence indicates that complement receptors on FDCs are important in both short- and long-term B-lymphocyte responses. Papamichail et al. demonstrated that retention of antigen–IgG immune complexes on FDCs was reduced upon depletion of C3 using cobra venom factor. Therefore it appears that immune complex deposition on FDCs is complement dependent. In addition, antibody production in vitro using FDCs demonstrates that antibody production is dependent on CD21/CD35. More recent studies on trafficking of complement-tagged immune complexes into skin draining LN using intravital imaging reported a two-step pathway. Initial binding of the C3 tagged complexes by CR3 expressed on sinus lining macrophages resulted in their transfer into the underlying B-cell compartment where they were “handed off” to naïve B cells in a CD21-dependent mechanism. Subsequently, the C3d-tagged complexes were stripped from the B-cell surface and internalized into the early endosomal compartment as they came in contact with CD21 receptors expressed on FDC. Thus, CD21 has a dual role in the B-cell delivery of C3d-tagged immune complexes from sinus macrophages to the stromal FDC.
Availability of Cr2-deficient mice has shed light on the importance of FDC-derived CD21/CD35 on humoral responses. Because FDCs are radioresistant, it was possible to generate chimeric mice that restricted CD21/CD35 expression to B lymphocytes by BM trans plantation. Ahearn et al. made chimeric mice with Cr2-deficient FDCs by transplanting wild-type BM (B-lymphocyte Cr2+/+) into lethally irradiated Cr2-deficient recipient mice (FDC- Cr2−/−). After secondary challenge with antigen, the chimeric mice failed to sustain high-level antibody production, suggesting that CD21/CD35 on FDCs is important for recall or memory responses. Fang et al. came to a similar conclusion regarding the importance of CD21/35 expression on FDC for a strong immune response.
CD21/CD35 do appear important for persistence of antibody titers, normal frequencies of memory B lymphocytes and plasma cells, and affinity maturation. Adoptively transferring memory B lymphocytes into recipient mice lacking FDC-derived CD21/CD35 demonstrated that complement receptors on recipient mice stroma were required for each of these elements of memory. Importantly, chimeric mice lacking CD21/CD35-bearing FDCs had severely impaired recall responses several months after transfer of memory B lymphocytes compared with wild-type recipients. Subsequent studies identified a mechanism explaining long-term retention of complement-opsonized immune complexes. Heesters et al. found that FDC internalize C3d-opsonized complexes via the CD21 receptor into a non-degradative cycling endosomal compartment that periodically exposes intact immune complexes on the cell surface for recognition by cognate B cells as discussed above. These studies suggest that CD21/CD35 on FDCs have an important role in long-term storage of antigen, thereby facilitating B-lymphocyte memory.
