A subset of circulating γδ T cell—Vδ—is depleted in both tuberculosis and HIV infections, irrespective of whether TB is active and whether the diseases are concurrent, Italian researchers have discovered.

Recent studies have suggested that γδ T lymphocytes have an important role in the immune response to Mycobacterium tuberculosis, but that the strength of the response varies according to the phase of infection and the immune status of the host.

CD4+, CD8+ and Vδ1 and Vδ2 T cell counts were analysed in the peripheral blood of 74 consecutive patients with TB, 20 of whom were co-infected with HIV. The results were compared with those from 39 blood donors and nine patients with symptomless HIV infection. The numbers of total lymphocytes and CD4+ cells were lower in the 54 TB patients not co-infected with HIV than in blood donors. But total γδ T cells and Vδ1 subsets were similar in both groups.

However, the percentage γδ T cells with the Vδ2 subset was significantly lower in TB patients than in either blood donors or patients with symptomless HIV infection. Responsiveness to the tuberculin skin test did not influence which γδ T cell subset predominated. The percentage of circulating Vδ2 cells was also lower in HIV positive patients, whether or not their TB was active, and comparable with levels in HIV negative patients.

The finding prompts the authors to ask whether the depletion of Vδ2 T cells might be explained by a pathological mechanism that is common to both infections.