Characterization of the protective T‐cell response generated in CD4‐deficient mice by a live attenuated Mycobacterium tuberculosis vaccine

Summary The global epidemic of tuberculosis, fuelled by acquired immune‐deficiency syndrome, necessitates the development of a safe and effective vaccine. We have constructed a ΔRD1ΔpanCD mutant of Mycobacterium tuberculosis (mc 2 6030) that undergoes limited replication and is severely attenuated in immunocompromised mice, yet induces significant protection against tuberculosis in wild‐type mice and even in mice that completely lack CD4 + T cells as a result of targeted disruption of their CD4 genes (CD4 –/– mice). Ex vivo studies of T cells from mc 2 6030‐immunized mice showed that these immune cells responded to protein antigens of M. tuberculosis in a major histocompatibility complex (MHC) class II‐restricted manner. Antibody depletion experiments showed that antituberculosis protective responses in the lung were not diminished by removal of CD8 + , T‐cell receptor γδ (TCR‐γδ + ) and NK1.1 + T cells from vaccinated CD4 –/– mice before challenge, implying that the observed recall and immune effector functions resulting from vaccination of CD4 –/– mice with mc 2 6030 were attributable to a population of CD4 –  CD8 – (double‐negative) TCR‐αβ + , TCR‐γδ – , NK1.1 – T cells. Transfer of highly enriched double‐negative TCR‐αβ + T cells from mc 2 6030‐immunized CD4 –/– mice into naive CD4 –/– mice resulted in significant protection against an aerosol tuberculosis challenge. Enriched pulmonary double‐negative T cells transcribed significantly more interferon‐γ and interleukin‐2 mRNA than double‐negative T cells from naive mice after a tuberculous challenge. These results confirmed previous findings on the potential for a subset of MHC class II‐restricted T cells to develop and function without expression of CD4 and suggest novel vaccination strategies to assist in the control of tuberculosis in human immunodeficiency virus‐infected humans who have chronic depletion of their CD4 + T cells.