Simian immunodeficiency virus is restricted to a subset of blood CD4⁺ lymphocytes that includes memory cells

HIV and the related simian immunodeficiency virus (SIV), which causes AIDS in macaques, infect only a small percentage of CD4⁺ lymphocytes at any point during the disease. We have identified three distinct cellular phenotypes within the CD4⁺ subpopulation in macaques, based on cell surface expression of CD44 and CD45R, which putatively represent successive stages of postthymic proliferation and functional maturation. Two of these subsets, CD44^hi CD45R⁺, which contained virtually all circulating cells in cycle, and CD44^hi CD45R^-, which was noncycling and has been linked to immunologic memory, were selectively depleted in SIV-infected animals at an asymptomatic stage of disease. To test whether SIV infection was restricted to cells with this phenotype in vivo, we used the polymerase chain reaction to sensitively detect SIV DNA in purified subpopulations of CD4⁺ lymphocytes. We found that SIV exclusively infected blood lymphocytes expressing high levels of CD44. Within this subset infection occurred not only in the fraction containing actively proliferating cells (CD45R⁺), but also in resting, putative memory cells (CD45R^-). These data directly demonstrate that cellular maturation stages of normal postthymic T lymphocyte differentiation are important factors in permitting lentivirus infection in vivo, and that noncycling, memory T cells may be a reservoir for SIV.