CD4⁺ memory T cells retain surface expression of CD31 independently of thymic function in patients with lymphoproliferative disorders following autologous hematopoietic stem-cell transplantation

High-dose chemotherapy with autologous hematopoietic stem-cell transplantation (AHSCT) causes severe and long-lasting immunodeficiency in patients with lymphoproliferative disorders. The thymus begins to restore the T-cell repertoire approximately from the sixth month post-transplant. We assessed the dynamics of post-transplant recovery of CD4⁺CD45RA⁺CD31⁺ T cells, “recent thymic emigrants” (RTEs), and a poorly described subtype of CD4⁺CD45RA⁻CD31⁺ T cells in 90 patients with lymphoproliferative disorders following high-dose chemotherapy with AHSCT. Relative and absolute counts of CD4⁺CD31⁺ naïve and memory T cells were evaluated before AHSCT, at the day of engraftment, and 6- and 12-month post-transplant. The pre-transplant count of CD4⁺CD45RA⁺CD31⁺ T cells was lower than in healthy controls, and did not reach donors’ values during the 12-month period. The pre-transplant number of CD4⁺CD45RA^–CD31⁺ T cells was higher than in healthy controls and was restored rapidly following AHSCT. Post-transplant mediastinal radiotherapy reduced counts of RTEs and elongated recovery period. Non-thymic tissue irradiation did not reduce this subset. The obtained data indicate that homeostatic proliferation may decrease the significance of CD31 expression on CD4⁺CD45RA⁺ T cells as a marker of RTEs, and suggest that evaluation of RTEs recovery by flow cytometry requires an accurate gating strategy to exclude CD31⁺ memory T cells.