BackgroundCD5, a member of the scavenger receptor cysteine-rich superfamily, is a marker for T cells and a subset of B cells (B1a). CD5 associates with T-cell and B-cell receptors and impair TCR signaling1 2 and increased CD5 is an indication of B cell activation. Furthermore, CD5 levels on CD8+ T cell splenocytes were significantly increased after TCR/CD3 stimulation using ex vivo treatment with anti-CD3/anti-CD28 MAbs compared to non-stimulated CD8+ T splenocytes.3 Previous studies have shown a correlation between CD5 and anti-tumour immunity where CD5 knockout mice inoculated with B16F10 melanoma cells had delayed tumour growth compared to wild type mice.4 In tumour-infiltrating lymphocytes (TILs) isolated from lung cancer patients, CD5 levels were negatively correlated with anti-tumour activity and tumour-mediated activation-induced T cell death,5 suggesting that CD5 could impair activation of anti-tumour T cells. However, the correlation between CD5 level expression and T cell activation and exhaustion in the tumour microenvironment and in peripheral organs is ill-defined and requires further investigation.MethodsWe determined CD5 levels in T cell subsets in different organs in mice bearing syngeneic 4T1 breast tumour homografts and assessed the relationship between CD5 and increased CD69 and PD-1 (markers of T cell activation and exhaustion) by flow cytometry.ResultsWe report that T cell CD5 levels were higher in CD4+ T cells than in CD8+ T cells in 4T1 tumour-bearing mice, and that high CD5 levels on CD4+ T cells were maintained in peripheral organs (spleen and lymph nodes). However, both CD4+ and CD8+ T cells recruited to tumours had reduced CD5 compared to CD4+ and CD8+ T cells in peripheral organs. In addition, CD5highCD4+ T cells and CD5highCD8+ T cells from peripheral organs exhibited higher levels of activation and associated exhaustion compared to CD5lowCD4+ T cell and CD5lowCD8+ T cell from the same organs. Interestingly, CD8+ T cells among TILs and downregulated CD5 were activated to a higher level, with concomitantly increased exhaustion markers, than CD8+CD5+ TILs.ConclusionsThus, differential CD5 levels among T cells in tumours and lymphoid organs can be associated with different levels of T cell activation and exhaustion, suggesting that CD5 may be a therapeutic target for immunotherapeutic activation in cancer therapy.AcknowledgementsThe author thanks Rene Figueredo and Ronak Zareardalan for their assistance in animal workEthics ApprovalThis study was approved by the Animal Use Subcommittee of the University of Western OntarioReferencesAzzam HS, et al., Fine tuning of TCR signaling by CD5. The Journal of Immunology 2001. 166(9): p. 5464–5472.Voisinne GA, Gonzalez de Peredo and Roncagalli R. CD5, an undercover regulator of TCR signaling. Frontiers in Immunology 2018;9:p. 2900.Alotaibi, F., et al., CD5 blockade enhances ex vivo CD8+ T cell activation and tumour cell cytotoxicity. European journal of immunology 2020;50(5): p. 695–704.Tabbekh, M., et al., Rescue of tumor-infiltrating lymphocytes from activation-induced cell death enhances the antitumor CTL response in CD5-deficient mice. The Journal of Immunology, 2011. 187(1): p. 102–109.Dorothée, G., et al., In situ sensory adaptation of tumor-infiltrating T lymphocytes to peptide-MHC levels elicits strong antitumor reactivity. The Journal of Immunology 2005;174(11): p. 6888–6897.
Delivery of IL-2 to the T Cell Surface Through Phosphatidylserine Permits Robust Expansion of CD8 T Cells
