Effects of soluble CD4 on simian immunodeficiency virus infection of CD4-positive and CD4-negative

A soluble form of the CD4 receptor (sCD4) can either enhance or inhibit the infection of cells by simian immunodeficiency virus (SIV) and human immuno deficiency virus. We investigated the basis for these varying effects by st udying the entry of three SN isolates into CD4-positive and CD4-negative ce lls expressing different chemokine receptors. Infection of CD4-negative cel ls depended upon the viral envelope glycoproteins and upon the chemokine re ceptor, with CCR5 and gpr15 being more efficient than STRL33. Likewise, enh ancement of infection by sCD4 was observed when CCR5- and gpr15-expressing target cells were used but not when those expressing STRL33 were used. The sCD4-mediated enhancement of virus infection of CD4-negative, CCR5-positive cells was related to the sCD4-induced increase in binding of the viral gp1 20 envelope glycoprotein to CCR5. Inhibitory effects of sCD4 could largely be explained by competition for virus attachment to cellular CD4 rather tha n other detrimental effects on virus infectivity (e.g., disruption of the e nvelope glycoprotein spike). Consistent with this, the sCD4-activated SIV e nvelope glycoprotein intermediate on the virus was long-lived. Thus, the ne t effect of sCD4 on SIV infectivity appears to depend upon the degree of en hancement of chemokine receptor binding and upon the efficiency of competit ion for cellular CD4.