High frequency of syncytium-inducing and CXCR4-tropic viruses among human immunodeficiency virus type 1 subtype C-infected patients receiving antiretroviral treatment

Abstract

Human immunodeficiency virus type 1 (HIV-1) subtype C viruses have been found to almost exclusively use the chemokine receptor CCR5 as a coreceptor for entry, even in patients with advanced AIDS. We have characterized subtype C virus isolates from 28 patients from Harare, Zimbabwe, 20 of whom were receiving antiretroviral treatment. Virus from 10 of the treated patients induced syncytium formation (SI virus) when cultured with MT2 cells. Only non-syncytium-inducing (NSI) virus was cultured from the peripheral blood mononuclear cells of the eight patients who had not received treatment. The majority of these subtype C SI viruses were capable of using both CCR5 and CXCR4 as coreceptors for viral entry, and the consensus V3 loop sequences from the SI viruses displayed a high net charge compared to those of NSI viruses. While those on treatment had reverse transcriptase (RT) and protease mutations, there was no clear association between RT and protease drug resistance mutations and coreceptor tropism. These results suggest that CXCR4-tropic viruses are present within the quasispecies of patients infected with subtype C virus and that antiretroviral treatment may create an environment for the emergence of CXCR4 tropism.

FIG. 1.

SI and NSI virus envelope sequences cluster with subtype C HIV. (a) env (C2-V5) RT-PCR product derived from low-passage-number virus stock was sequenced and aligned with CLUSTAL W. The phylogenetic tree was made from gap-stripped sequences of 529 nt each. Bootstrap values from 100 data sets that have a greater than 90% frequency are indicated on the tree. Boxed samples indicate patients who were not receiving treatment. Filled triangles indicate samples with R5-X4 coreceptor usage, while the open triangle indicates X4 usage only. GenBank accession numbers for subtype C reference sequences are as follows: C.BR.92, U52953; C.BW.96, AF110967; C.IN.95, AF067155; C.ET.86, U46016. The accession numbers for the other reference sequences are given in the figure. (b) TC-03 sequence 1 and sequence 2 and TC-28 sequence 1 and sequence 2 correspond to two distinct yet closely related groups of viruses. env V3 loop sequences from PBMC virus stock and MT2 or SupT1 virus stock were compared to consensus C sequence (Con C).

FIG. 2.

Subtype C SI viruses have an increase in the number of basic amino acids compared to that in the env V3 loop consensus sequence (Con C) as well as a higher average net charge. The net charge of the sequence is derived from the summation of charges from positive and negative amino acids. A potential glycosylation site is underlined. Sequences for TC-03 and TC-28 correspond to virus that grew from MT2 cells; the remaining sequences are from low-passage-number PBMC stock.