Differential susceptibility of quiescent CD4⁺ lymphocytes to syncytial-inducing and non-syncytial-inducing isolates of HIV-1

It is generally believed that quiescent CD4⁺ T cells are not susceptible to HIV-1 infection. However, infection of unstimulated peripheral mononuclear cells by syncytial-inducing (SI) viruses has been shown to be much more efficient than with nonsyncytial-inducing (NSI) viruses. This suggested that SI, CXCR4-tropic viruses may be able to infect quiescent CD4⁺ T cells. We studied the infection of highly purified quiescent CD4⁺ T cells by SI and NSI viruses. In this article we show that although NSI viruses failed to significantly infect quiescent cells, SI viruses consistently infected these cells and produced viruses upon cellular activation by interleukin-2,2 to 7 days after initial infection.To examine whether the difference was the result of viral or host factors, we purified CCR5⁺ quiescent CD4⁺ T cells and showed that these cells can be infected by dual tropic (R5X4) but not by R5 virus. This indicated that CCR5⁺ quiescent T cells were also susceptible to HIV-1 infection, and the failure of NSI, CCR5-tropic viruses to infect quiescent cells may be due to some intrinsic properties of these viruses.