Simian immunodeficiency viruses have been isolated from four species of monkey, the 'captive' macaque and mangabey and the 'feral' African green monkey and mandrill. While none of these viruses is a replica of HIV-1, the macaque and mangabey viruses represent correct genetic models for HIV-2, possessing exactly the same complement of genes. Recently a lentivirus has been identified in two wild chimpanzees (Pan troglodytes troglodytes) in Gabon, west equatorial Africa, and isolated from one of them. This virus is referred to as SIVCPZ. Sera from these animals cross reacted with all the HIV-1 proteins including the envelope glycoproteins. Here, we describe the molecular cloning and sequencing of an infectious proviral clone of SIVCPZ. The overall genetic organization was the same as that of HIV-1, but phylogenetic analysis revealed that the sequence was more divergent than any HIV-1 sequence reported so far. The vpu gene product, found only in the type 1 viruses, was particularly different (64% divergent to HIV-1BRU) suggesting that the SIVCPZ represents a distinct subtype. These findings indicate that there is a larger pool of simian lentiviruses than previously suspected and revives debate as to the origins of HIV-1.

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Antigen-binding T and B lymphocytes were studied by combined autoradiography and immunofluorescence; mouse spleen lymphocytes binding the antigens, [125I]MSH or [125I]TIGAL, were incubated with rhodamine-labeled anti-Ig reagents or with a rhodamine-labeled IgG fraction of anti-θ serum. B cells were identified as Ig+ or θ-, T cells as Ig- or θ+. It was found that: (a) 20% (1–2 mo after priming) to 30% (3.5–4 mo after priming) of the antigen-binding cells were T cells. (b) The range of antigen molecules bound by B and T cells was similar. (c) Binding of antigen to B and T cells was inhibited by polyvalent anti-Ig, anti-µ, or anti-L reagents. Binding to T cells was more readily inhibited than to B cells. Normal rabbit serum, antimouse lymphocyte serum, or anti-θ did not inhibit antigen binding. (d) When Ig at the surface of B cells was induced, by noninhibiting concentrations of anti-Ig reagents, to redistribute into polar caps and the cells subsequently exposed to [125I)antigen under noncapping conditions, the [125I]antigen silver grains were distributed in caps superimposed on the Ig fluorescent cap. Of crucial importance, antigen was found in cap in the same proportion of T cells as B cells. Significant capping of antigen receptors was not induced in B or T cells with normal rabbit serum or by anti-Ig reagents absorbed with mouse Ig. The main conclusions of this series of experiments using direct visualization of antigen-binding B and T lymphocytes is that T cells have antigen-specific receptors, probably of IgM nature, and that the number of these receptors appears to range in the order of thousands.