Lymphocyte subpopulation analysis and improvement in SLC19A1-deficient patients following folic supplementation. (A) Analysis of T cell populations revealed mature CD4⁺ cells in P1 and P2, with an unusually high number of terminally differentiated CD4 T cells present in P2. (B) Analysis of B cell differentiation confirmed the presence of class-switched IgD-CD27⁺ cells in both patients (P1, P2) as well as mature activated IgD⁺CD27⁺ B cells, albeit at lower levels in P1. Results from 2 controls (Ctrl1, Ctrl2), both healthy, are shown for comparison. (C-D) P1 and P2 exhibited reduced subpopulations of CD24^hi unswitched memory cells compared with controls. (E) Normal levels of CD24^hi transitional B lymphocytes were identified in both patients, although 10% to 20% of these cells failed to express CD21. (F) Platelet counts before and during (on) folic acid supplementation and during periods when treatment was paused. (G) Summary of improvements observed in P1 and P2 before and during (on) supplementation.