One of the key downregulators of T cell activation is CD152 (CTLA-4). Mice genetically deficient in CD152 (cd152–/– mice) develop massive expansion of both CD4+ and CD8+ T cells as well as increased numbers of splenic Ig-secreting cells and serum Ig levels. To determine the dependence of the lymphoproliferation and B cell hyperactivity on MHC class II (MHCII), MHCII-deficient (mhcii–/–) cd152–/– mice were generated. Compared to that in their mhcii+/+ counterparts, expansion of CD4+ cells in mhcii–/–cd152–/– mice was markedly attenuated. Nonetheless, expansion of CD8+ cells was identical in both sets of mice, demonstrating that the effects of CD152 deficiency on CD4+ cells can quantitatively be dissociated from those on CD8+ cells, and pointing to a critical downregulatory role for CD152 in MHCII-independent CD8+ cell activation in vivo. B cell hyperactivity also developed in mhcii–/–cd152–/– mice, albeit in a manner less rapid and less intense than that in their mhcii+/+ counterparts, demonstrating an underlying MHCII-independent diathesis to B cell dysregulation and pointing to a critical downregulatory role for CD152 in MHCII-independent B cell activation in vivo. When human DQ8 was introduced as a transgene into mhcii–/–cd152–/– mice, B cell hyperactivity was restored to levels observed in mhcii+/+cd152–/– mice, pointing to a critical downregulatory role for CD152 in MHCII-dependent B cell activation in vivo superimposed upon its downregulatory role on MHCII-independent B cell activation.