The antigen-specific immune response in HIV sero-positive individuals is depressed or absent. This may be due in part to abnormal co-operation between T lymphocytes and antigen presenting cells (APC). We have isolated from the peripheral blood of healthy heterosexuals and patients with persistent generalized lymphadenopathy (PGL) and AIDS, cells of low density (LDC) with dendritic morphology. These cells are known to be potent APC. The expression of two cell surface antigens on these cells, namely 63D3 (a monocyte related antigen) and Class II antigens was examined. LDC from controls and patients with benign, non-progressive PGL (type A) were found to show biphasic expression of Class II antigens. By contrast, the high intensity Class II expression seen on a small proportion of 63D3 negative cells from controls and patients with PGL type A was absent in patients with PGL type B (showing subtle signs of progressive immunodeficiency) and AIDS. The loss of this population of dendritic cells was reflected in the absence of stimulator activity in autologous and heterologous mixed lymphocyte reactions. Thus, it is possible that the loss of these dendritic cells may contribute to the profound immunological abnormalities seen in AIDS
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