Intrinsic Survival Capacity of Cord Blood T cells is heterogeneous and Can Be Promoted by Repeated Low-Doses of Recombinant Interleukin-7,

Umbilical cord blood (CB) transplantation is known to be associated with delayed and defective immune reconstitution, in part because recent thymic emigrants (RTEs), the most common subset among CB T cells, have limited intrinsic survival. Interleukin-7 (Il-7) has been reported to increase the initial recovery of the graft-derived T-cells compartment. The aim of this study was to investigate CB-derived T-cell survival and to define a minimal effective dose of recombinant human IL-7 (rIL-7), considering that in addition to its anti-apoptotic effect, IL-7 may enhance immune reactions that occur in the host, including uncontrolled acute GVHD.

Fifteen CB were obtained immediately after normal-term delivery, using the same procedure as for CB banking. T cells were isolated within 12 hours by negative magnetic bead sorting and cultured for 2 weeks either directly or after being frozen in order to recapitulate clinical procedures. Unstimulated T cell cultures were conducted in parallel in medium (RPMI supplemented with 10% normal AB serum) alone or supplemented with a range of rIL-7concentrations added either only once at day 0 (100 to 1000 pg/mL) or every day (10 to 100 pg/mL). Cell viability was assessed by flow cytometric scatter analysis and staining with 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] and propidium iodide.

In basic culture conditions, the majority of T cells had died over 2 weeks, but there was a marked heterogeneity in cell survival, as proportions of viable T cells varied from 2% to 50% (median 15%) at day 6 of culture. Interestingly, the same intrinsic characteristics of survival were observed in T cells that underwent beforehand a freezing-thawing procedure.

In cultures supplemented with rIL-7, we confirmed the efficacy of rIL-7 in maintaining CB T cell survival. A minimal dose of 20 to 50 pg/mL of IL-7 added daily was sufficient to induce the prolonged survival of CB T cells, with more than 95% of viable cells at day 6. Noteworthy, these concentrations allowed prolonged survival of CD T cells without inducing any cell proliferation, as shown by the absence of CFSE dilution. Similar results were observed comparing CB cells that had been frozen or not.

Repeated low-doses of rIL-7 are required to preserve the survival capacity of CB T cells without inducing their proliferation. These results could represent a framework for clinical studies aiming at improving T cell reconstitution following allogeneic CB transplantation. Repeated administration of low-doses of rIL-7 might be especially useful in patients with low serum levels of IL-7 after conditioning.

No relevant conflicts of interest to declare.