Blocking suppressor cells boosts TB vaccine protection

Tuberculosis (TB) remains one of the deadliest infectious diseases worldwide, and current vaccination with intradermal (i.d.) BCG offers only limited protection. Researchers led by Manfred B. Lutz revisited an alternative approach using whole Mycobacterium tuberculosis (Mtb) material: subcutaneous (s.c.) immunization with heat-killed Mtb. Previous work from this group had shown that this form of immunization increases a particular immune cell population called monocytic myeloid-derived suppressor cells (M-MDSC) in mice. Because M-MDSC are known to suppress immune responses, the team hypothesized that an Mtb-based immunization that drives M-MDSC might actually weaken protection against a later mycobacterial challenge. To test that idea, the researchers immunized mice with heat-killed Mtb, then exposed them to BCG infection to evaluate how well the immunization protected against a live mycobacterial challenge. The experiments were designed to track both protective outcomes and the balance between immune activation and suppression, focusing on key cell types and specific interventions to alter M-MDSC levels.

The experiments produced an unexpected result: mice vaccinated with heat-killed Mtb were protected against a subsequent BCG infection and showed elevated frequencies and activation of dendritic cells (DC) and mycobacteria-specific T cells, even though high frequencies and suppressor activity of M-MDSC were present. To probe whether reducing M-MDSC could further improve outcomes, the team used two approaches described in the abstract. First, genetic ablation of CCR2+ monocytic cells reduced the frequency of Mtb-induced M-MDSC. Second, a pharmacological intervention with all-trans retinoic acid (ATRA) also lowered M-MDSC frequencies. Both strategies enhanced frequencies and activation of DC and CD4+ T cells and led to decreased bacterial loads in the lung and spleen. These findings were observed in the mouse model and compared immunological readouts (cell frequencies and activation) together with bacterial burden to show that removing or reducing M-MDSC can tip responses toward stronger mycobacterial control.

Taken together, the results reported by Manfred B. Lutz and colleagues offer a fresh perspective on using heat-killed Mtb as a vaccine approach. They show that heat-killed Mtb can induce protective immunity against BCG despite simultaneously inducing vaccine-associated M-MDSC, and that targeted reduction of those suppressive cells can further improve protection. The genetic ablation of CCR2+ monocytic cells establishes that monocytic populations contribute to the suppressive environment, while pharmacological depletion with all-trans retinoic acid (ATRA) provides a practical way to shift the balance toward protective immunity. The authors suggest that M-MDSC depletion by ATRA could be considered as an adjunct host-directed therapy alongside TB vaccines in humans, aiming to enhance vaccine efficacy by reducing unwanted immune suppression while preserving or boosting dendritic cell and T cell activation.