CR1(CD35) AND CR2(CD21) COMPLEMENT C3 RECEPTORS ARE EXPRESSED ON NORMAL HUMAN THYMOCYTES AND MEDIATE INFECTION OF THYMOCYTES WITH OPSONIZEDHUMAN-IMMUNODEFICIENCY-VIRUS

The present study demonstrates that the C3b receptor CR1 (CD35) and th e C3dg/Epstein-Barr virus receptor CR2 (CD21) are expressed by 25% and 70% of normal human thymocytes, respectively. The expression of CR2 e xtends to both CD1(+) and CD1(-) cells in the thymus. Two subsets of C R2(+) thymocytes were defined expressing low and high density of the r eceptor. The CR2(++) subset represented 20% of CR2(+) thymocytes and c o-expressed the CR1 receptor. CR2(++) thymocytes expressed an immature CD1(dull), CD3(-), CD4(dull), CD8(-), CD7(++) phenotype and included a subpopulation of large cells expressing CD34. Twenty percent of thym ocytes expressed the CD21 epitope defined by monoclonal antibody BU32, which is involved in the binding of CD23 to CD21. These observations provide a basis for a role for CD21 in the proliferation and different iation of thymocytes at early stages of maturation. The functionality of CR1 and CR2 on thymocytes was evidenced by the ability of the recep tors to mediate infection of cells with complement-opsonized human imm unodeficiency virus (HIV). The results may be relevant to the immunopa thogenesis of HIV infection.