IL6 and neoplasm: However, the expression of MHC molecules on CTVT cells is important, with CTVT cells switching from an MHC-negative phenotype during tumour growth to MHC-positive during tumour regression (see below for further discussion).56,60,61 Cytokines are also thought to play a role.62 Higher concentrations of the pro-inflammatory cytokines IL-6 and IFN-γ are detected in ex vivo cultures of tumour-infiltrating lymphocytes from regressing tumours compared with growing tumours and the presence of these cytokines increases cytotoxicity of NK cells to CTVT cells in vitro62 (Fig.2).