The Importance of TGFβ to Skin Tissue Resident Memory T Cells

CD8+ tissue resident memory (Trm) T cells are long-lasting immune cells retained at peripheral tissue sites where they can mount a rapid and robust local response to previously encountered microbial pathogens. In the epidermal compartment, Trm cell development is critically dependent on transforming growth factor β (TGFβ). This cytokine, produced in a latent form, is activated in skin by keratinocyte-expressed αvβ6 and αvβ8 integrins, and skin CD8+ T cells exposed to active TGFβ show upregulated expression of the surface molecule CD103. CD103 can bind E-cadherin on epithelial cells ultimately promoting skin retention of Trm T cells.4 In an article published earlier this year in Immunity, Hirai et al. use a series of sophisticated mouse experiments to determine the source of TGFβ and its role in supporting epidermal Trm cell residence.

To establish whether continued exposure to TGFβ was necessary for long-term maintenance of CD8+ Trm cells in the skin, the authors generated CD8+ ovalbumin-specific transgenic T cells (OT-I) engineered to express a tamoxifen (TAM)-inducible Cre-recombinase that ablates the gene encoding TGFβ receptor type 2 (Tgfbr2). These cells were adoptively transferred into recipient mice who were subsequently infected via a skin scarification method with vaccina virus expressing ovalbumin (VV-OVA). 42 days after infection—a timepoint by which Trm cell populations would have formed and differentiated—TAM was administered to induce deletion of Tgfbr2 and 30 days later epidermal sheets were collected. The authors found that post-infection ablation of Tgfbr2 led to a significantly reduced number of epidermal Trm cells.

Following this, Hirai and colleagues generated mouse models in which the gene encoding TGFβ (Tgfb1) could be inducibly excised from keratinocytes or CD8+ T cells. Loss of Tgfb1 expression in CD8+ T cells, but not keratinocytes, effected Trm cell numbers. Importantly, Trm cells lacking Tgfb1 did not negatively impact Tgfb1+ cells in close proximity, leading the authors to conclude that Trm cell maintenance depends on autocrine TGFβ.

Unlike in other tissues, Trm cell differentiation in the skin does not depend on secondary encounter with cognate antigen. Indeed, non-specific inflammation caused by local administration of agents such as the sensitizing hapten 2,4-dinitrofluorobenzene (DNFB) can efficiently recruit T effector cells to the skin where they can differentiate into bystander Trm cells. Bystander Trm cells also depend on TGFβ as, like their antigen-specific counterparts, this population is absent in mice lacking keratinocyte expression of TGFβ-activating integrins. The authors thus sought to understand the relative importance of active TGFβ to these different Trm populations. For these experiments, OT-I cells were transferred to wild type mice following which the left flank was infected with VV-OVA while the right flank was topically treated with DNFB. Both treatments recruited and maintained similar numbers of OT-I Trm cells. However, when αvβ6 and αvβ8 integrin-mediated TGFβ activation was blocked post-infection, only bystander Trm cell numbers were significantly reduced. Importantly, bystander Trm cells exposed during the differentiation phase to VV-OVA or in which TGFβ receptor signaling was genetically enforced were better able to persist following αvβ6 and αvβ8 inhibition. These data indicate that Trm cells that have encountered cognate antigen are more resilient than bystander Trm cells in epidermal environments where TGFβ is limited.

To compare antigen-specific and bystander Trm cell fitness in a TGFβ-competitive environment, the authors used the same OT-I adoptive transfer and flank-treatment regimen as above, but also treated a distal site (the abdomen) with the antigenically distinct sensitizing hapten oxazolone to generate oxazolone-specific CD8+ T cells. Following oxazolone challenge, these cells were recruited to flank skin where they would compete with Trm cells for local TGFβ. Reduction in Trm cell numbers was observed at both the VV-OVA and DNFB-treated sites, but bystander Trm populations were consistently lower than those of antigen-specific Trm cells. In OT-I cells where TGFβ receptor signaling was genetically enforced oxazolone challenge no longer significantly impacted Trm cell numbers. Thus, newly recruited CD8+ T cells decrease Trm cell retention with a preference toward displacing bystander Trm cells. When Trm cells are given a genetic advantage for accessing TGFβ, however, they are better able to compete with newly recruited CD8+ T cells. Together, this indicates an important role for TGFβ in regulating Trm cell populations and their persistence in the epidermal compartment.

Finally, the authors carried out transcriptomic profiling to understand the mechanism driving the competitive advantage antigen-specific Trm cells have over bystander Trm cells. While differences were not observed in TGFβ related pathways, bystander Trm cells did show increased expression of histone gene variants associated with chromosome silencing. This suggests that bystander Trms may be less epigenetically poised to quickly respond to changes in environmental TGFβ levels than antigen-specific Trm cells.

Hirai and colleagues not only demonstrated that CD8+CD103+ Trm cell residency is supported by autocrine TGFβ, but also revealed competition for active TGFβ as a mechanism for preferentially maintaining antigen-specific Trm populations over bystander Trm populations in the epidermal niche. Although this research focused on skin, it is possible these findings could be extended to CD103+ Trm cells found in other tissues such as the gastrointestinal tract or lung. The mechanisms by which this molecule is maintaining memory programming and whether this cytokine is of equal importance to CD103- Trm cell populations remain to be determined. Overall, this increased understanding of the role TGFβ in Trm cell maintenance could enable the rational design of vaccines and immunotherapies capable of enhancing tissue-specific immunity.