Ho. from individual infected animals carry gp120s containing comparable changes (specific amino acid deletions, substitutions, and loss of N-linked glycosylation sites), primarily within the V1 and/or 5-Methylcytidine V2 regions of gp120. In vivo, the principal target of human immunodeficiency virus type 1 (HIV-1) is the CD4+ T cell. Over time, virus-induced elimination and/or dysfunction of this T-lymphocyte subset, whether caused directly or indirectly, leads to clinical disease in infected individuals (10). Like all other lentiviruses, HIV-1 is also able to infect macrophage, a property recognized since the beginning of the AIDS epidemic (21, 50). The pathogenic CORIN consequences of HIV-1-infected macrophages is best exemplified by the AIDS dementia complex in which viral RNA expressed in microglia within the central nervous system may cause severe neuronal injury leading to encephalopathy (24). In addition, because of their reported resistance to the cytopathic effects elicited by HIV-1 and long life span, tissue macrophages have also been considered to be an important reservoir of virus (36). In this regard, macrophages have been 5-Methylcytidine reported to be a prominent source of virus during the late stages of disease, especially in conjunction with opportunistic infections (35). HIV-1 contamination of macrophages in vivo has been logistically difficult to study. For example, it is not currently known whether monocyte precursors are initially infected in the bone marrow and only begin to produce virus after their migration to and differentiation in specific tissues or whether they become infected after they reach their final tissue of residence. Current understanding about the dynamics of virus production by tissue macrophage is similarly limited: are progeny virions generated as a result of de novo infections or by the continuous release of particles by long-lived cells? Consequently, most present knowledge about HIV-1 contamination of macrophage accrues from an in vitro surrogate: monocyte-derived macrophage (MDM). MDM have been useful for identifying so-called macrophage-tropic (M-tropic) HIV-1 strains. M-tropic strains infect MDM and primary CD4+ T lymphocytes but not most human T-cell lines in vitro, fail to induce syncytium formation, and can be recovered from infected persons during all phases of their disease (7, 13, 42). Prototypic M-tropic strains of HIV-1 use the CCR5 chemokine receptor for cell entry (1, 11). From studies of highly pathogenic SHIVs that irreversibly and systemically deplete CD4+ T lymphocytes in rhesus monkeys within weeks of virus inoculation, we previously reported that tissue macrophages are able to sustain high levels of plasma viremia ( 106 RNA copies/ml) in the virtual absence of CD4+ T cells (16). It was subsequently found that viral variants, circulating in the plasma during the macrophage phase of SHIV infections, carried gp120 V2 changes (specific double amino acid deletions and the loss of a conserved glycosylation site) (19). Some of these SHIV variants had also acquired the capacity to infect alveolar macrophages (AM). In the present study, the gp120 structure, cell tropism, and coreceptor utilization properties of macrophage-phase simian immunodeficiency virus/HIV chimeras (SHIVs), isolated directly from lymphoid tissues, were examined. In contrast to plasma virus, the tissue-associated SHIVs bore envelope glycoproteins that were genetically more heterogeneous and contained specific changes primarily within V1 and V2. Small molecule coreceptor-targeted inhibitors, specific for CCR5 or CXCR4, were used to assess 5-Methylcytidine the chemokine receptor usage by (i) the starting highly pathogenic, T-cell-deleting SHIVs and the (ii) late-phase M-tropic SHIV variants. 5-Methylcytidine Blockade of CXCR4 potently suppressed contamination of rhesus monkey PBMC by both viruses, whereas CCR5 targeted inhibitors had little if any effect. Contamination of macaque AM by M-tropic SHIVs was also suppressed by CXCR4 not by CCR5 specific inhibitors. Thus, the acquisition of macrophage tropism by SHIVs, present in late-phase monkeys, is not accompanied by a switch in chemokine coreceptor usage. MATERIALS AND METHODS Virus. The propagation and properties of the highly pathogenic SHIVDH12R.