Temporal development of B and T cell areas in the neonatal rabbit appendix. (90.14)

The rabbit appendix is organized structurally into multiple B cell follicles separated, at their sub-apical regions, by well-defined T cell areas. We found, by RNA in situ hybridization, that the B cell chemoattractant CXCL13 (B-lymphocyte chemoattractant, or BLC) is highly expressed in the B cell follicles. In addition, we found that the T cell chemoattractant CCL21 (secondary lymphoid tissue chemokine, or SLC) is highly expressed in the T cell areas. The localized expression of these chemoattractants likely organizes the B and T cell areas of the appendix. Surprisingly, however, we found that the localized expression of CXCL13 and CCL21 differs dramatically in early neonatal appendix. At two days after birth, CXCL13 expression was confined to the appendix domes and CCL21 expression was localized in a continuous layer significantly beneath the domes. Over subsequent days, as B cells accumulated in the domes and began proliferating, developing follicles gradually distended downward through the layer of CCL21 expression, ultimately generating the characteristic organization of B and T cell areas seen in the adult appendix. This developmental process might be important in establishing an environment supportive of B cell proliferation and antibody repertoire diversification, which occur in the rabbit appendix between 3 and 6 weeks of age.