CD4 T cells are important regulators of the immune system and are vital for mounting a strong immune response against viral infections. Human cytomegalovirus (HCMV) is known to be a strong modulator of the innate as well as adaptive immune responses. In this study, we found that HCMV directly inhibited proliferation of CD4 T cells and rendered them unresponsive to immunological stimuli. This effect was not observed when CD4 T cells were treated with herpes simplex virus-1/2 or measles virus. When stimulated with phytohemagglutinin, concanavalin A, or phorbol myristate acetate, HCMV-treated T cells were unable to proliferate, revealing an ability of HCMV to inhibit CD4 T cell response. Furthermore, HCMV also prevented proliferation of leukemic T-cell lines. HCMV-treated CD4 T cells expressed the activation markers CD45RO and CD69, were not apoptotic and produced decreased levels of the cytokines IL-4, IFN-γ and TNF-α, compared to untreated controls. The inhibitory effect of HCMV on CD4 T cell proliferation was not mediated by HCMV gH, gB or other immunogenic glycoproteins, since intravenous immunoglobulins or gB- or gH-specific neutralizing antibodies did not prevent the suppression of T-cell proliferation. Our observations show that HCMV inhibits CD4 T cell function with potential clinical consequences for both humoral and cell-mediated immune responses.
