Changes in L-Selectin Expression by T Lymphocytes after Extracorporeal Photochemotherapy.
Authors: Ramirez Manuel , Gonzalez-Vicent Marta , Perez-Martinez Antonio , Sevilla Julian , Madero Luis , Diaz Miguel A. 
Published: October 2019

Extracorporeal photochemotherapy (ECP) is an emerging treatment modality for steroid-refractory acute and chronic graft versus host disease (GVHD). The mechanisms by which ECP works are still not fully understood, and modulation of dendritic cell subpopulations, a shift of cytokine profile from Th1 to Th2 and an increase of T-cell regulatory (Treg) cells have been related to the ECP beneficial effect. Changes on T-lymphocyte subsets other than Treg have been reported after ECP (Biol Blood Marrow Transplant2006; 12(1 Suppl 2):22–30.). We analyzed the effect of ECP on the T lymphocyte subsets of sixteen children receiving this form of therapy. Steroid refractory GVHD was defined as failure to respond to 2 mg/Kg/day of methylprednisolone after 5 days (acute GVHD) or failure to respond to steroids or flare of disease activity upon tapering (chronic GVHD). ECP was performed by an alternative approach to that of the classical UVAR XTS system. This alternative method is based on a continuous-flow cell separator (COBE Spectra) that allow to process more mononuclear cells in a smaller volume and it is less time-consuming, both important advantages for pediatric patients. We studied the L-selectin (CD62L) and the CD45RA expression on CD4 and CD8 lymphocytes by flow cytometry. This combination allowed us to distinguish naive (TN, CD62L+CD45RA+), central memory (TCM, CD62L+CD45RA+), effector memory (TEM, CD62L+CD45RA+), and terminal differentiated T cells (TT, CD62L+CD45RA+), as described in Science2000;290:92–97. We compared the proportion of each of these four subpopulations, as well as the L-selectin positive and L-selectin negative ones, in samples collected from peripheral blood before the first (PRE) and after the last (POST) ECP procedures. Statistical analyses were done by the Wilcoxon signed-rank test. Results are shown in the following tables:

CD4 SUBSETS & ECP PRE POST p value TN 6.58 ± 2.39 6.18 ± 2.96 0.2003 TCM 58.17 ± 4.49 43.85 ± 4.78 0.0268 TEM 34.64 ± 4.51 46.86 ± 4.89 0.0356 TT 0.6 ± 0.18 3.11 ± 1.46 0.1704 CD62Lpos 64.76 ± 4.4 50.03 ± 5.45 0.0268 CD62Lneg 35.23 ± 4.4 49.97 ± 5.45 0.0268 CD8 SUBSETS & ECP PRE POST p value TN 16.95 ± 3.94 12.53 ± 4.05 0.2343 TCM 28.8 ± 4.95 12.27 ± 2.86 0.0013 TEM 46.62 ± 5.79 51.4 ± 4.22 0.1477 TT 11.17 ± 3.34 23.52 ± 4.79 0.0105 CD62Lpos 45.75 ± 6.1 24.8 ± 5.34 0.0174 CD62Lneg 53.8 ± 6.07 74.92 ± 5.31 0.0174

The clinical outcome of the patients was always positive, with more than 50% achieving complete remission. The proportion of L-selectin expressing T lymphocytes significantly diminished after ECP, both in CD4 and in CD8 cells. The reason for these changes are currently unknown. L-selectin is an important T-cell homing receptor for T-cell entry into lymph nodes via high endothelial venules. Expression of CD62L is rapidly lost following T-cell receptor activation, leading to exit from the lymph node into the periphery and sites of inflammation. CD62Lneg and CD62Lpos also differ in their functional abilities, such as cytokine secretion and cytolytic potential. Our results suggest that ECP may have an impact in the trafficking patterns of T lymphocytes, redirectioning T cells from lymphoid to extralymphoid organs. In addition, ECP was associated to a redistribution of the pool of non-naive T lymphocytes.