Investigating the role of CD226 on CD4⁺ T cells in the context of cancer

Activating receptors on T cells, such as CD226 (DNAM-1), play an important role for anti-cancer responses. Tumour cells can downregulate CD226 on CD8⁺ T cells in mouse and human tumours and that the success of immune checkpoint blockade in melanoma patients correlates with the presence of CD226⁺ CD8⁺ T cells. While there is a growing recognition for the role of CD4⁺ T cells in anti-tumour immunity, the role of CD226 in CD4⁺ T cells functions remains unclear. In both mouse and human, CD226 expression is upregulated during activation and correlates with the functionality of CD4⁺ T cells. Furthermore, a rapid downregulation of CD226 is observed upon ligation with its ligand, CD155. In tumour infiltrating CD4⁺ T cells, we also observed reduced CD226 surface expression. A mutation at the tyrosine 319 (Y319) residue showed resistance to CD155-driven downregulation. Mechanistically, CD155 induced Y319 phosphorylation, which led to Cbl-b mediated ubiquitination, internalisation, and ultimately proteasomal degradation of CD226. To further investigate the role of CD226 in tumour immunity, the herpes simplex glycoprotein D (gD) was utilised as a model antigen, because it harbours an epitope (gD₃₁₅–₃₂₇) recognized by CD4⁺ T cells from the transgenic gDT-II mice. In-vitro co-culture studies using CRISPR/Cas9 generated CD226KO CD4⁺ gDT-II cells, showed limited cytokine production and activation as compared to WT controls. Hence, suggesting a role for CD226⁺ CD4⁺ T cells in anti-tumour function. In summary, our findings provide insights to CD226 dynamics and its contribution to anti-tumour capacity of CD4⁺ T cells. While therapeutic implications of CD226 on CD4⁺ T cells remain to be fully explored, our study prompts future investigation into its functional and potential therapeutic significance.