Human macrophage and T cell lines were stably transfected with HIV-1 wild-type Tat or Tat mutants in the cysteine-rich region displaying trans-dominant negative effects on HIV-1 life cycle. The expression of HLA class I and class II molecules was not affected by wild-type Tat. Tat mutants, instead, profoundly down-regulated in a dose-dependent fashion the expression of class II, but not of class I, in both cell types by acting at the transcriptional level. Down-regulation was manifested on constitutive and IFN-gamma-induced class II gene expression and did not correlate with reduced transcription of the AIR-1 gene product CIITA, the major transcriptional activator of class II genes, indicating that Tat mutants did not act by inhibiting AIR-1 gene expression. Class II down-modulation had important functional implications in macrophages, as both antigen processing and presenting capacity were inhibited. These results represent the first evidence that a modified HIV-1 Tat product can act as a potent immunosuppressor by inhibiting the HLA class II expression necessary for triggering both cellular and humoral responses against pathogens. The use of these HIV-1 Tat mutants also discloses new opportunities to investigate the molecular mechanisms underlying the coordinate HLA class II gene transcription.