CTLA4 and B-cell chronic lymphocytic leukemia: Checkpoint molecule CTLA-4 was markedly upregulated in all analyzed lymphocyte subsets (CD4+, CD8+, and CD19+) in both patient groups versus HV (p < 0.001), with the strongest expression observed again in CLL+SID (p = 0.048–<0.001 vs. CVID; Figure 1M–O).