LOWER PREVALENCE AND INCIDENCE OF HIV-1 SYNCYTIUM-INDUCING PHENOTYPE AMONG INJECTING DRUG-USERS COMPARED WITH HOMOSEXUAL MEN

Objective: To study the prevalence, incidence and predictive value for progression to AIDS of the HIV-1 syncytium-inducing (SI) phenotype in HIV-infected injecting drug users (IDU) compared with HIV-infected ho mosexual men. Design: Two prospective cohort studies on HIV-1 infectio n among IDU and homosexual men. Methods: HIV-infected IDU (n=225) and homosexual men (n=366) without AIDS were studied from March 1989 throu gh December 1993. Data on laboratory markers, including the presence o f SI variants, demographics, behavioural characteristics and clinical events were collected at every visit. Results: At baseline, SI variant s were detected in 4% of IDU and 17% of homosexual men. During the stu dy period 18 IDU and 68 homosexual men switched from non-SI to SI phen otype (4-year cumulative incidence, 14.6 and 28.4%, respectively) befo re AIDS diagnosis. Among participants with a documented date of HIV in fection the cumulative incidence of SI was lower among IDU than homose xual men (4-year cumulative incidence, 6.2 and 20.7%, respectively). A t AIDS diagnosis, 21% of all AIDS cases among IDU had the SI phenotype compared with 54% among homosexual men. In both risk groups an accele rated CD4 decline was found after the non-SI-to-SI switch. The SI phen otype appeared to be a predictor of AIDS (multivariate relative hazard , 5.33), independent of CD4 cell count and p24 antigen at baseline. In the multivariate time-dependent analysis, the relative hazard of SI p henotype decreased considerably, which is consistent with the hypothes is that the effect of SI phenotype on progression to AIDS is mediated by CD4 cell count. Conclusion: The SI phenotype is associated with acc elerated CD4 decline and progression to AIDS in both risk groups. The remarkable lower prevalence and incidence of the SI phenotype among ID U may implicate a difference in pathogenesis and natural history of HI V infection linked to transmission group.