Infusions of HSV-TK Engineered Donor Lymphocytes Effectively Protect Patients Undergoing Haploidentical Stem Cells Trasplantation (HSCT) from Infectious Mortality.

T-cell depletion (TCD) of allogeneic stem cells transplantation (HSCT) is the most powerful approach to overcome HLA barriers in patients with high risk malignancies, lacking a conventional donor. However, the prolonged immune deficiency resulting from TCD significantly impairs the outcome of HSCT from a haploidentical family donor (haplo-HSCT). In a phase I-II clinical trial (protocol MMTK007), we showed that the add-back of suicide-gene transduced donor lymphocytes (TK+ cells) provides early immune-reconstitution (IR), crucial to avoid opportunistic infections and disease relapse. While the achievement of an early and sustained IR significantly reduced the incidence of infectious complications and mortality, the kinetic of abatement of viral reactivations differed in different patients, suggesting variability in the repertoire, function and persistence of viral-specific T cells. To gain insights on the activity of the post-transplant immune system, we focused on immune responses to cytomegalovirus (CMV) and Ebstein Barr virus (EBV) as clinical paradigm of an effective immune system. In our trial 29 patients (median age 52y) were transplanted for high-risk leukemia. Seventeen pts received TK-DLI starting at day +42, 14 pts obtained prompt immune reconstitution with CD3⁺ >100/mcl at day +86 (median) from SCT and day +21 from TK-DLI. Twelve/14 experienced CMV reactivation, 3 reactivation/patient (median), each reactivation required 14 days (median) of pre-emptive treatment with foscarnet to achieve a complete clearance; in 2/38 events, a CMV antigenemia persistent during foscarnet treatment was cleared by ganciclovir administration. In this series, the last CMV reactivation requiring pre-emptive treatment happened at day +84 from SCT and day +19 from TK-DLI. Nine/14 experienced EBV reactivation, 1 reactivation/patient (median), 6 patients required treatment with rituximab (375 mg/mq weekly) for 3 cycles (median). In 3 cases we documented EBV lymphoproliferative disease, that were completely controlled with the treatment. The proportion of lymphocytes committed to produce IFN-γ upon CMV or EBV stimulation normalized within the first 3 months post transplant. The analysis of CMV and EBV T-cell response showed a prevalent host-restriction pattern, suggesting active modulation of the T-cell repertoire by the host environment. The TCR-Vβ repertoire progressively changed from oligoclonal into polyclonal and normalized by 1 year after transplant. The progressive acquirement of a protective, host-restricted, anti-viral response, highly correlated with a decline in the occurrence of any infectious event, that were nearly abrogated by 6th month post transplant. The overall cumulative infectious mortality beyond day +100 post transplant was 12,5% in TK treated patients demonstrating the efficacy of this strategy in building a protective immune system.