The decrease in T-cell numbers in HIV-infected individuals is caused by

• A. Lysis of host T cells by replicating virus.
• B. Fusion of the T cells to form syncytia.
• C. Killing of the T cells by HIV-specific cytotoxic T cells.
• D. All of the above.

Answer: (D)

HIV causes progressive loss of CD4+ T cells through multiple pathways. Infected T cells can die directly from viral replication, a direct cytopathic effect that reduces the number of healthy T cells. The virus also promotes fusion of infected T cells with neighboring cells, forming syncytia that are dysfunctional and eventually die, further decreasing T-cell counts. Additionally, HIV-specific cytotoxic T lymphocytes target and kill infected T cells, contributing to depletion as the immune system works to control the infection. Since all of these processes occur, the overall decline in T-cell numbers reflects the combined impact of direct viral cytopathy, syncytium formation, and CTL-mediated killing. That’s why all of these mechanisms together best explain the decrease in T cells during HIV infection.