TIGIT and neoplasm: Finally, RT can increase the expression of Programmed Death Ligand 1 (PD-L1) on the surface of tumor and immunosuppressive myeloid cells [30] as well as the expression of the T cell immunoreceptor with Ig and ITIM domains (TIGIT), a co-inhibitory receptor expressed on CD8+ T cells, natural killer cells, Tregs and T follicular helper cell [31,32,33].