Mycobacterium tuberculosis (Mtb) subverts host defenses to persist in macrophages despite immune pressure. CD4 T-cells can recognize macrophages infected with a single bacillus in vitro . Under identical conditions, CD8 T-cells inefficiently recognize infected macrophages and fail to restrict Mtb growth, although they can inhibit Mtb growth during high burden intracellular infection. We show that high intracellular Mtb numbers cause macrophage death, leading other macrophages to scavenge cellular debris and cross-present the TB10.4 antigen to CD8 T-cells. Presentation by infected macrophages requires Mtb to have a functional ESX-1 type VII secretion system. These data indicate that phagosomal membrane damage and cell death promote class I MHC presentation of the immunodominant antigen TB10.4 by macrophages. Although this mode of antigen-presentation stimulates cytokine production that we presume would be host beneficial; killing of uninfected cells could worsen immunopathology. We suggest that shifting the focus of CD8 T-cell recognition to uninfected macrophages would limit the interaction of CD8 T-cells with infected macrophages and impair CD8 T-cell mediated resolution of tuberculosis.

Containment of Mycobacterium tuberculosis (Mtb) infection requires T cell recognition of infected macrophages. Mtb has evolved to tolerate, evade, and subvert host immunity. Despite a vigorous and sustained CD8(+) T cell response during Mtb infection, CD8(+) T cells make limited contribution to protection. Here, we ask whether the ability of Mtb-specific T cells to restrict Mtb growth is related to their capacity to recognize Mtb-infected macrophages. We derived CD8(+) T cell lines that recognized the Mtb immunodominant epitope TB10.4(4-11) and compared them to CD4(+) T cell lines that recognized Ag85b(240-254) or ESAT6(3-17). While the CD4(+) T cells recognized Mtb-infected macrophages and inhibited Mtb growth in vitro, the TB10.4-specific CD8(+) T cells neither recognized Mtb-infected macrophages nor restricted Mtb growth. TB10.4-specific CD8(+) T cells recognized macrophages infected with Listeria monocytogenes expressing TB10.4. However, over-expression of TB10.4 in Mtb did not confer recognition by TB10.4-specific CD8(+) T cells. CD8(+) T cells recognized macrophages pulsed with irradiated Mtb, indicating that macrophages can efficiently cross-present the TB10.4 protein and raising the possibility that viable bacilli might suppress cross-presentation. Importantly, polyclonal CD8(+) T cells specific for Mtb antigens other than TB10.4 recognized Mtb-infected macrophages in a MHC-restricted manner. As TB10.4 elicits a dominant CD8(+) T cell response that poorly recognizes Mtb-infected macrophages, we propose that TB10.4 acts as a decoy antigen. Moreover, it appears that this response overshadows subdominant CD8(+) T cell response that can recognize Mtb-infected macrophages. The ability of Mtb to subvert the CD8(+) T cell response may explain why CD8(+) T cells make a disproportionately small contribution to host defense compared to CD4(+) T cells. The selection of Mtb antigens for vaccines has focused on antigens that generate immunodominant responses. We propose that establishing whether vaccine-elicited, Mtb-specific T cells recognize Mtb-infected macrophages could be a useful criterion for preclinical vaccine development.

Although memory CD4 T cells are critical for effective immunity to pathogens, the mechanisms underlying their generation are still poorly defined. We find that following murine influenza infection, most effector CD4 T cells undergo apoptosis unless they encounter cognate Ag at a defined stage near the peak of effector generation. Ag recognition at this memory checkpoint blocks default apoptosis and programs their transition to long-lived memory. Strikingly, we find that viral infection is not required, because memory formation can be restored by the addition of short-lived, Ag-pulsed APC at this checkpoint. The resulting memory CD4 T cells express an enhanced memory phenotype, have increased cytokine production, and provide protection against lethal influenza infection. Finally, we find that memory CD4 T cell formation following cold-adapted influenza vaccination is boosted when Ag is administered during this checkpoint. These findings imply that persistence of viral Ag presentation into the effector phase is the key factor that determines the efficiency of memory generation. We also suggest that administering Ag at this checkpoint may improve vaccine efficacy.