Background: The chemokine receptors CCR5 and CXCR4 are major coreceptors for HIV-1 in vivo. At least 12 other chemokine receptors or related molecules support infection of CD4⁺ cell lines by various HIV-1 strains. However, there is little current evidence to indicate that these alternative co-receptors contribute significantly to viral replication in vivo or pathogenesis. Thus, the capacity of different HIV-1 strains to exploit alternative coreceptors for infection of cell lines does not truly reflect coreceptor-use in vivo. Here, we have tested if different human primary cell cultures express functional alternative coreceptors. The capacity of a naturally expressed alternative coreceptor to support HIV-1 infection may provide strong evidence for a role in vivo.

Methods: We used primary brain microvascular endothelial cells (BMVECs) and adult astrocytes that don’t express CCR5 or CXCR4. While BMVECs expressed CD4, an Adenovirus vector was used to express CD4 on astrocytes. A panel of HIV-1, HIV-2, and SIVs were titrated on each cell culture. Infection was assessed 3 days later by immunostaining for viral antigens. Infection of PHA/IL-2 treated peripheral blood mononuclear cells (PBMCs) was tested by measuring reverse transcriptase activity in culture supernatant.

Results: A subset of HIV and SIVs were identified that could exploit an alternative co-receptor on astrocytes and BMVECs. These included about 25% of R5X4 HIV-1 strains tested. The identified viruses were assessed for their capacity to infect CCR5^- PBMCs where CXCR4 was blocked by AMD3100. Of 2 HIV-1 strains tested, 1 replicated efficiently, while low level replication was detected for a 2nd. BMVEC infection and AMD3100-resistant replication in PBMCs were blocked by the chemokine vMIP-I implicating a vMIP-I receptor as the coreceptor involved. Although vMIP-I binds CCR8, CXCR6, and GPR1, mRNA expression for these potential coreceptors (detected by RT-PCR) did not correlate with BMVEC or PBMC infection via the alternative coreceptor.

Conclusions: An unidentified vMIP-I receptor acts as a functional co-receptor for a subset of HIV and SIVs. This co-receptor is active on PBMCs, representing major cell targets for HIV in vivo. The HIV-1 strains identified were R5X4 viruses, thus implying that the alternative co-receptor contributes to a broadening of cell tropism late in disease.