Background:  CD25⁺ lymphoid cells are a subset of CD4⁺ regulatory T cells (Treg) that play a major role in immune regulation. Few studies have addressed changes of Treg in lymphoid tissue in HIV-1 infection. This study investigates the influence of HAART and structured therapy interruption on the distribution of CD25⁺/CD4⁺ Treg cells in lymphoid tissue.

Method:  From 9 HIV-1-infected patients, we obtained 22 biopsies before treatment (baseline, n = 5), after a mean of 24 months of HAART (n = 8), and after 17 months of discontinuing HAART after 5 cycles of structured therapy interruption (n = 9). Lymphoid tissue viral load was determined in frozen samples by using the NucliSens HIV-1 RNA QT assay. Immunohistochemical studies were carried out using anti-CD25 (Novocastra, 4C9) and anti-CD4 (Novocastra, 1F6) antibodies. Positive cells were counted in interfollicular areas and the mean value per HPF was calculated.

Result:  Mean CD4 T cell counts in peripheral blood at baseline, HAART and structured therapy interruption periods were 684, 757, and 784 x 106 copies/L, respectively. Mean plasma viral load at baseline and HAART and structured therapy interruption periods were 4.28, 2.3, and 3.13 log₁₀ copies/mL, respectively. Tonsil biopsies before treatment showed an absence of lymphoid follicles (stage III, 5 of 5 biopsies). After the HAART period, we observed an improvement of immuno-architecture with recovery of follicular structures (stage I, 4 of 8), which was maintained after structured therapy interruption (stage I, 6 of 9). Immunohistochemical analysis demonstrate an increase of CD25⁺ Treg cell population in the interfollicular areas (1.4, 7.72, and 12.82 cells per HPF at baseline, and the HAART and structured therapy interruption periods, respectively; p = 0.01). The CD4⁺ cells in lymphoid tissue also increased (181, 290, and 368 cells per HPF at baseline, and the HAART and structured therapy interruption periods, respectively; p = 0.051). Lymphoid tissue viral load decreased significantly after HAART and structured therapy interruption (6.7, 2.7, and 3.5 log₁₀ copies/mg of tissue; p = 0.036). The recovery of immuno-architecture after HAART and structured therapy interruption was significantly associated with a drop in lymphoid tissue viral load and an increase of CD4⁺ positive cells and CD25⁺ Treg cells in lymphoid tissue (p <0.02). The drop in lymphoid tissue viral load correlated with the increase in CD25⁺ Treg cells in lymphoid tissue (p = 0.05).

Conclusions:  The number of CD4⁺CD25⁺ Treg cells in lymphoid tissue of HIV-infected patients significantly improves after HAART and structured therapy interruption, and this recovery correlates with a decrease of the lymphoid tissue viral load and recovery of immuno-architecture. These results suggest that CD25⁺ Treg may play a role in the control of viral load and may be a useful index of immunological recovery on lymphoid tissue biopsies from HIV patients.