Proposed model of association between productive HIV replication in CD4⁺ T cells and cytokine expression, activation, apoptosis, and cell proliferation. According to this model, HIV either preferentially infects or up-regulates Th1 cytokines. The presence of type 1 cytokines, such as IL-2, leads to the up-regulation of Bcl-2, which protects the cells from apoptosis and promotes cell cycle entry. However, owing due to viral proteins, as is well documented, the productively infected cells become arrested in the G₂/M phase of the cell cycle.