CTLA4 and cancer: To investigate the impact of ABCs on mRNA-LNP vaccination, we next analysed the immune response to the mRNA BNT162b2 COVID-19 vaccine in patients with variable expansion of ABCs, including healthy controls (HC, n = 10), patients with cancer treated with ICB (ICB, n = 19) and patients with NFKB1 or CTLA4 haploinsufficiency (IEI, n = 9).