Primary HIV-1 infection of human CD4+T cells passaged into SCID mice leadsto selection of chronically infected cells through a massive Fas-mediated autocrine suicide of uninfected cells

We have recently shown that a human CD4(+) T cell line (CEMSS) acquires the permissiveness to M-tropic strains and primary isolates of HIV-1 after tra nsplantation into SCID mice. This permissiveness was associated with the ac quisition of a memory (CD45RO+) phenotype as well as of a functional CCR5 c oreceptor. In this study, we have used this model for investigating in vivo the relationships between HIV-I infection, apoptosis and T cell diferentia tion. When an in vivo HIV-1 infection was performed, the CEM cell tumors gr ew to a lower extent than the uninfected controls, CEM cells explanted from uninfected SCID mice (ex vivo GEM) underwent a significant level of sponta neous apoptosis and proved to be CD45RO+, Fas+ and Fas-L+, while Bcl-2 expr ession was significantly reduced as compared to the parental cells. Acute H IV-1 infection markedly increased apoptosis of uninfected ex vivo CEM cells , through a Fas/Fas-L-mediated autocrine suicide/fratricide, while parental cells did not undergo apoptosis following viral infection, The susceptibil ity to apoptosis of ex vivo CEM cells infected with the NSI strain of HIV-1 , was progressively lost during culture, in parallel with the loss of Fas-L and marked changes in the Bcl-2 cellular distribution. On the whole, these results are strongly reminiscent of a series of events possibly occurring during HIV-1 infection. After an initial depletion of bystander CD4+ memory T cells during acute infection, latently or chronically infected CD4+ T ly mphocytes are progressively selected and are protected against spontaneous apoptosis through the development of an efficient survival program. Studies with human cells passaged into SCID mice may offer new opportunities for a n in vivo investigation of the mechanisms involved in HIV-1 infection and C D4+ T cell depletion.