CD4 and viral infectious disease: In conclusion, our data support an immunopathogenic model in which the effect of HIV upon the thymus occurs through at least three mechanisms: (i) direct viral lysis which induces T-cell depletion with a preferential effect on CD4+-expressing thymocytes, (ii) bystander activation which involves all thymopoietic stages, resulting in high turnover, higher susceptibility to viral infection and exhaustion of regenerative capacity, (iii) enrichment of Treg in the thymus possibly leading to the suppression of pathogen-specific immunity and contributing to lack of control of HIV.