Background: We have previously shown that CD4 inhibits HIV replication by blocking Env function. This block can be relieved by the action of Nef, Env, and Vpu, which down-modulate the viral receptor and allow the synthesis of infectious particles. Defective nef alleles have been associated with slow or nonprogression to disease. However, it is controversial which functions of Nef are important for pathogenesis. Progression to disease correlates with the appearance of Nef alleles with strong in vitro CD4 down-modulation activity. We hypothesize that late Nef alleles may relieve the inhibitory effect imposed by CD4 more efficiently than alleles from early stages of disease, thereby conferring a replicative advantage to viruses growing in cells with high levels of CD4.

Methods and Results: To study the role of CD4 down-modulation we have used an in vitro system in which HIV viruses are grown in Jurkat cell lines with different levels of surface CD4. We have analyzed a panel of nef alleles isolated from HIV-infected patients at different stages of disease. These nef alleles have been cloned into NL4.3 constructs and the infectivity and replicative capacity of the viruses were evaluated. HIV viruses with Nef alleles from late stages of infection (AIDS patients [pts]) down-modulate efficiently the CD4 receptor, and produce viral particles with infectivity higher than viruses with alleles from asymptomatic pts. The differences in infectivity between late and early viruses disappear in T-cells with low levels of surface-CD4, regardless of the origin of the nef allele or the ability of the virus to down-modulate the viral receptor. Interestingly, these differences are also diminished when viruses are analyzed in target cells with high levels of CD4.

Conclusions: Our results show that HIV variants with strong CD4 down-modulation activity that emerge as progression to disease occurs overcome more efficiently the inhibitory effect of CD4, and thus replicate more efficiently in high-CD4 cells. Other investigators have shown that late-stage Nef alleles conferred to the virus enhanced ability to replicate in primary lymphocytes. The ability of Nef to enhance replication in primary lymphocytes appears to correlate with its ability to down-modulate CD4. Taken together, these results suggest that CD4 down-modulation may be an important determinant for HIV pathogenesis, and suggest that interfering with this function might alter the course of disease.