Baseline circulating MAIT cell count and colitis in Cohort 3.
We sought to determine if baseline MAIT cell proportion could be used as a peripheral biomarker to predict IN-COL in a second distinct cohort in a blinded manner. Our validation of gating analysis using Cohort 3 found high correlation between the two results (R2=0.989; p = 1.3x10-72; Supplementary Figure 3a-b). Absence of non-specific background staining for CD161 and TCR Vα7.2 was demonstrated in additional experiments using isotype control antibodies (see Supplementary Figure 2bii). We confirmed in Cohort 3 that melanoma patients who develop IN-COL (N=14) have a lower proportion of circulating MAIT cells compared with healthy volunteers (Figure 4c). In contrast to Cohort 2, where baseline MAIT cells was a useful discriminator between the IN-COL and IN-NAE groups, in Cohort 3 the IN-NAE patients also had a lower proportion of circulating MAIT cells at baseline compared to healthy volunteers and there was no significant difference between IN-COL and IN-NAE MAIT cell proportions (Figure 4c).