The functional effect of CD1c responsive T cells in human Tuberculosis (TB)

Abstract

Tuberculosis (TB) is an infectious disease with one of the highest rates of mortality, and remains a global health pandemic. Despite major research programmes over the last two decades, outcomes of novel interventions for TB have been disappointing; and the consistent emergence of multi-drug resistant strains continue to pose a challenge. Thus, a deeper understanding of the host-pathogen interaction is urgently required for the development of more efficacious treatments. Mycobacterium Tuberculosis (Mtb), the causative agent of TB, owes its high virulence to its lipid-rich cell wall, many of which can trigger a host-mediated immune response during infection. Mycobacterial derived lipids are presented by the CD1 family of proteins, which include CD1c. During the natural course of Mtb infection, CD1 proteins bind lipid antigens and present them to T cells. An increase in CD1c-restricted Mtb-lipid specific T cells in the peripheral blood of TB patients has been documented. Importantly, the majority of CD1c-restricted T cells are also autoreactive as they recognise CD1c in the absence of exogenous lipid antigens. Hence, CD1c-autoreactive T cells recognise CD1c proteins when they are bound to endogenous self-lipid antigens. Nevertheless, the exact role that CD1c-autoreactive T cells play in the host response to TB is yet to be elucidated. In this study, we aimed to identify whether CD1c-autoreactive T cells have an as of yet undetermined anti-TB function. Henceforth, we optimised cellular in vitro assays to facilitate the detection, isolation, and expansion of CD1c-autoreative T cells before investigating their anti-TB functions. We explored various methods of expanding CD1c-autoreactive T cell populations from pan T cells derived from human blood, and determined that THP1-CD1c mediated expansion was the most effective method of enriching these populations. Following expansion with THP1-CD1c, we employed CD1c-tetramers to isolate CD1c-autoreactive T cells. T cell reactivity to CD1c was validated through a combination of tetramer studies and functional assays. Importantly, CD1c-autoreactive T cells were significantly cytotoxic towards Mtb infected THP1-CD1c target cells, but not towards uninfected target cells, nor towards target cells that did not express CD1c, irrespective of their infection status. In addition, while CD1c-autoreactive T cells released cytokines in response to uninfected CD1c⁺ target cells in an autoreactive manner, they produced significantly enhanced amounts of a variety of anti-microbial cytokines including TNF-α, IFN-γ, GM-CSF, IL-4, IL-5, and IL-13 in response to Mtb infected CD1c⁺ targets. Moreover, phenotypical studies confirmed that our CD1c-autoreactive T cells were CD4⁺ and are polyclonal as they expressed αβ- or hybrid δ/αβ-TCRs. Lastly, to better understand the TCR repertoire of CD1c-autoreactive T cells we completed sequencing studies which identified eight unique TCRs, four of which were αβ-TCRs, and four were γδ-TCRs. Altogether, the data highlights that while these T cells are overtly self-reactive towards CD1c, they also respond even more strongly to a specific human infection such as Mtb. Hence, these findings suggest a specific functional role for CD1c-autoreactive T cells in the immune response to TB, and sheds light on a potential protective mechanism that could be manipulated to design future therapeutic interventions.

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Identifiers

ORCID for Sahar Farag: orcid.org/0000-0003-2360-7094

ORCID for Paul Elkington: orcid.org/0000-0003-0390-0613

ORCID for Salah Mansour: orcid.org/0000-0002-5982-734X

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