CD5 + B cells are preferentially expanded in rabbit appendix: The role of CD5 in B cell development and selection

Although only a small proportion of mouse and human B cells are CD5 +, most adult rabbit B cells express CD5. However, CD5 was not detectable on the majority of B cells in neonatal appendix 1 and 3 days after birth. Cell trafficking studies demonstrated that CD5 + and CD5 − CD62L + B cells from bone marrow migrated into appendix. There, CD5 + B cells were preferentially expanded and predominated by ∼2 weeks of age. In mutant ali/ ali rabbits, VHa2 + B cells develop through gene conversion-like alteration of rearranged VH genes upstream of deleted VH1a2. Correlated appearance of individual CD5 + germinal centers and VHa2 + B-cells in mutant appendix suggests that CD5 binding positively selects cells with a2 + framework regions that bind CD5. Following negative and positive selection, cells with diversified rearranged heavy- and light-chain sequences exit appendix, migrate to peripheral tissues and constitute the preimmune repertoire of CD5 + B cells that encounter foreign antigens.