Infectious cellular load in human immunodeficiency virus type 1 (HIV-1)-infected individuals and susceptibility of peripheral blood mononuclear cells from their exposed partners to non-syncytium-inducing HIV-1 as major determinants for HIV-1 transmission in homosexual couples.

To study risk factors for homosexual transmission of human immunodeficiency virus type 1 (HIV-1), we compared 10 monogamous homosexual couples between whom transmission of HIV-1 had occurred with 10 monogamous homosexual couples between whom HIV-1 transmission had not occurred despite high-risk sexual behavior. In the group of individuals who did not transmit virus, peripheral cellular infectious load was lower and the CD4+ T-cell counts were higher than in the group of transmitters. HIV-1 RNA levels in serum did not differ between transmitters and nontransmitters. Compared with peripheral blood mononuclear cells (PBMC) from normal healthy blood donors, 8 of 10 nonrecipients and only 3 of 8 recipients had PBMC with reduced susceptibility to in vitro infection with non-syncytium-inducing (NSI) HIV-1 variants isolated from either their respective partners or an unrelated individual. No difference in susceptibility was observed for infection with a syncytium-inducing variant. Among the individuals who had PBMC with reduced susceptibility, five nonrecipients and one recipient had PBMC that were equally or even less susceptible to NSI variants than PBMC that had low susceptibility and that were derived from healthy blood donors that were heterozygous for a 32-bp deletion in the CCR5 gene (CCR5 delta32). Three of these individuals (all nonrecipients) had a CCR5 delta32 heterozygous genotype themselves, confirming an association between low susceptibility to NSI variants and CCR5 delta32 heterozygosity. All three recipients with less susceptible PBMC had partners with a high infectious cellular load; inversely, both nonrecipients with normally susceptible PBMC had partners with a very low infectious cellular load. These results suggest that a combination of susceptibility of target cells and inoculum size upon homosexual exposure largely determines whether HIV-1 infection is established.