Short-Term Ionomycin Exposure Activates Naive Murine T-Cells and Induces a Rapid Phenotypic Shift to Memory T-Cell Status: Potential for Use as a Method To Reduce GvHD Activity of Allogeneic T-Cells.
Authors: Hossain Mohammad , Giver Cynthia R. , Waller Ned 
Published: April 2020

Abstract
We are investigating methods to reduce the graft-versus-host disease (GVHD) potential of donor T-cells while retaining graft-versus-leukemia (GVL) activity in allogeneic HSCT. Previous investigations by our group and others in have shown that naive CD4 T-cells induce severe acute GVHD, while memory CD4 T-cells do not induce GVHD but retain GVL activity in murine transplant models. These findings have led to studies for the development of methods to increase the number of memory T-cells available for transplant. The calcium ionophore, ionomycin, is a T-cell activating agent and mitogen. By increasing intracellular Ca2+ levels, ionomycin is induces T-cell activation through signaling mechanisms including phospholipase C activation, hydrolysis of phosphoinositides, and activation of protein kinase C. Differences in memory and naive T-cell responses to ionomycin have been attributed to resistance of memory T-cells to increases in Ca2+. Memory T-cells lack intracellular Ca2+ stores, and are also resistant to influx of Ca2+. Brief low dose ionomycin exposure (20min, 2μM) of T-cells, leading to increased density of naive T-cells, has previously been exploited as a method for separating memory and naive T-cells by Percoll gradient separation. Since ionomycin exposure induces T-cell activation through native Ca2+ dependent signaling mechanisms, we hypothesized that ionomycin-treated T-cells would shift to an activated/memory T-cell phenotype. Murine splenic T-cells were treated with 1.3μM ionomycin for 4hr. Memory and naive T-cell subsets and activation markers were analyzed by flow cytometry. 75% and 85% of untreated CD4 and CD8 T-cells, respectively, had the CD62L+ naive phenotype. These numbers were dramatically reduced to 7% and 17% after ionomycin exposure, representing a shift to the memory T-cell phenotype. Viability of T-cells was not significantly affected. The majority of remaining CD62L+ naive T-cells expressed activation markers CD25 and CD69. The fraction of CD4+CD25+Foxp3+ regulatory T-cells was also determined by intracellular staining of the transcription factor and co-expression of surface markers. CD4+CD25+Foxp3+ regulatory T-cells represented 4% of untreated CD4 T-cells and 3% of ionomycin-treated CD4 T-cells. While ionomycin has been used for many years in studies of T-cell activation, to our knowledge this is the first demonstration of a rapidly-induced shift of naive T-cells to a memory phenotype. A pilot experiment was conducted testing the GVHD activity of ionomycin-treated splenocytes (SP) in B6→ (B6 × Balb/C)CB6F1 recipients. 5 × 106 T-cell depleted bone marrow cells (TCD-BM) were transplanted along with 10 × 106 treated or untreated SP. Mice that received untreated SP all died from acute GvHD by 34 days after transplant, while all recipients of ionomycin-treated SP survived until the experiement was terminated at day 49 (average weight loss was 25%, data not shown). Continuing experients will refine the dose to further reduce GVHD symptoms and also test GVL activity of the treated cells. Treatment of donor T-cells with ionomycin may represent a clinically applicaple method to engineer donor lymphocyte infusions that are safer for HSCT patients.
Figure 1. Survival of CB6F1 recipients after transplant with 5 million B6 TCD-BM and 10 million B6 splenocytes that were either untreated or stimulated ex-vivo with a combination of PMA, ionomycin and brefeldin-A for 4 hours. 5 recipient animals per group. The experiment was terminated at day 49. Figure 1. Survival of CB6F1 recipients after transplant with 5 million B6 TCD-BM and 10 million B6 splenocytes that were either untreated or stimulated ex-vivo with a combination of PMA, ionomycin and brefeldin-A for 4 hours. 5 recipient animals per group. The experiment was terminated at day 49.