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N aturally SIV-infected sooty mangabeys (SMs) remain asymptomatic despite high virus replication. Elucidating the mechanisms underlying AIDS resistance of SIV-infected SMs may provide crucial information to better understand AIDS pathogenesis. In this study, we assessed the determinants of set-point viremia in naturally SIV-infected SMs, i.e., immune control of SIV replication versus target cell limitation. We depleted CD4⁺ T cells in 6 naturally SIV-infected SMs by treating with humanized anti-CD4 mAb (Cdr-OKT4A-huIgG1). CD4⁺ T cells were depleted almost completely in blood and BM and at variable levels in mucosal tissues and LNs. No marked depletion of CD14⁺ monocytes was observed. Importantly, CD4⁺ T cell depletion was associated with a rapid, significant decline in viral load, which returned to baseline level at day 30–45, coincident with an increased fraction of proliferating and activated CD4⁺ T cells. Throughout the study, virus replication correlated with the level of proliferating CD4⁺ T cells. CD4⁺ T cell depletion did not induce any changes in the fraction of Tregs or the level of SIV-specific CD8⁺ T cells. Our results suggest that the availability of activated CD4⁺ T cells, rather than immune control of SIV replication, is the main determinant of set-point viral load during natural SIV infection of SMs.