Background: IFNg production by CD4 T cells has been thought to be critical for immunity against Mycobacterium tuberculosis (Mtb), however recent studies show that IFNg-producing CD4 T cells are more effective at preventing dissemination than controlling Mtb in the lung. Because optimal control of Mtb infection requires direct interactions between CD4 T cells and Mtb-infected cells presenting cognate antigen on MHCII, we sought to determine the location of CD4 T cell antigen recognition and IFNg production in the Mtb-infected lung.
Methods: We infected mice with an ultra-low dose (ULD) of Mtb (1-3 CFU), a model developed in our lab which results in well-circumscribed granulomas that recapitulate many features of human Mtb granulomas. Using immunohistochemistry and quantitative imaging we examined their lungs 35 days later for phenotypic and spatial analysis of T cell receptor (TCR) signaling (using IRF4) and IFNγ production. We tested the antigen specificity of these responses with an adoptive transfer of both Mtb-specific and OVA-specific control CD4 T cells into ULD Mtb-infected mice. To assess the role of TGFβ signaling on T cell localization and function, we performed the same analysis in mice lacking the TGFβ receptor (TGFβR) on T cells.
Results: Within Mtb-infected lungs, many T cells localize near Mtb cells and undergo TCR signaling. Despite this, we found very few cells producing IFNɣ within the granuloma (Fig 1). In our adoptive transfer experiment, both cell types infiltrated the granuloma. The Mtb-specific, but not OVA-specific, T cells had active TCR engagement though only a small fraction of these cells produced IFNɣ, and this IFNɣ was diminished near Mtb (Fig 2). Conversely, in the TGFβR conditional knockout we found increased IFNɣ production that was highest within the granuloma (Fig 3).
Conclusion: Despite ongoing TCR stimulation in T cells, IFNγ production is restricted in areas where cognate interactions are most likely to occur. TGFβ plays a critical role in mediating this effect, as T cells lacking the receptor can produce more IFNγ near infected cells. These findings help explain why IFNγ-producing T cells have limited capacity to control pulmonary Mtb infection and could guide new strategies for vaccine and immunotherapeutic development.
C. Plumlee, None
M. Gerner, None
K. Urdahl, None