Background:  Multi-drug-resistant HIV-1 variants are generally thought to be less fit than wild type virus. However, transmission of multi-drug-resistant HIV-1 is well documented and in some cohorts with increased prevalence. We reported a case of recently transmitted multi-drug-resistant HIV-1 infection associated with dual tropism and rapid clinical progression. Here, we describe the replication characteristics of bulk and clonal isolates from this case and compare these to a panel of transmitted multi-drug-resistant and wild type viruses.

Methods:  Biological clones (3~4 clones per case) were obtained from peripheral blood mononuclear cells (PBMC) by limiting dilution co-culture (1:5-fold dilution) from this patient, multi-drug-resistant-1, 3 recently infected, drug-naive patients (multi-drug-resistant-2~4), and 2 cases without drug resistance (wild type-1, 2). Co-receptor usage was examined by MT-2 assay and Ghost cell assay. Viral replication rates were calculated from daily p24 values in the supernatant, and cytotoxicity was evaluated by the reduction in CD4⁺ T cell number in PHA/IL-2 stimulated PBMC culture.

Results:  Infectivity of propagated biological clones or bulk culture isolates was highest in multi-drug-resistant-1. Up-slope (log₁₀/day) of viral replication was extremely high in multi-drug-resistant-1 (1.30±0.30: mean±SD for clonal isolates) when compared to multi-drug-resistant-2~4 (0.75±0.08) and wild type-1, -2 (0.82±0.03). Two multi-drug-resistant transmissions demonstrated SI variants with varying syncytium-forming activity. The bulk isolate of multi-drug-resistant-1depleted CD4⁺ T cells very rapidly compared to the other viruses tested.

Conclusions:  X4+ (SI ++) multi-drug-resistant clones and bulk virus derived from patient multi-drug-resistant-1 grow to higher titers at more rapid rates. The bulk isolate was highly cytopathic in vitro. Transmitted viruses multi-drug-resistant-2~4 and wild type-1, -2 demonstrated comparable in vitro infectivity and growth. Understanding mechanisms of compensation for multi-drug-resistant and determinants of these described characteristics are critical in advancing understanding of HIV-1 pathogenesis.