Thymosin Alfa 1 Administration Improves Immune Reconstitution and Decreases Infection-Related Mortality After HLA-Matched Sibling T Cell-Depleted Stem Cell Transplantation.

Dendritic cells (DCs) play a pivotal role in initiating and modulating immune responses to pathogens. Data from experimental allogeneic bone marrow transplantation showed impaired DC function post-transplant as DCs produced mostly TNF-alfa with low amounts of IL-10, IL-12 and IFN-alpha, did not express co-stimulatory molecules upon activation with fungal antigens and were unable to phagocytose fungi. In vitro and in vivo data demonstrated Thymosin alpha 1 (T alpha 1), a naturally occurring peptide, induced DC activation, maturation and differentiation. T alpha 1 stimulated phagocytosis and functional maturation of murine pulmonary DCs upon exposure to Aspergillus conidia, accelerated lymphoid cell recovery and activated protective Th1-dependent resistance to infection. In a murine model of allogeneic bone marrow transplantation with lethal pneumonia caused by an A. fumigatus challenge, T alpha 1 promoted balanced Th1/Treg immunity and protected mice from invasive aspergillosis [1, 2]. The activation of innate immune system by T alpha 1 was mediated by distinct Toll-Like Receptor (TLR) signalling culminating in the activation of the p38 MAPK/NF-kB pathway. More recently, transcription profile of DCs exposed to T alpha 1 revealed a number of genes modulated by T alpha 1, including those involved in the regulation of the canonical/noncanonical NF-kB pathway in response to cellular stress and homeostasis. Accordingly, we designed a phase I/II clinical trial to determine the safety and efficacy of T alpha 1 administration in 30 recipients (12/30 with active disease at transplant) of HLA-matched sibling T cell-depleted stem cell transplants. Patients aged 20–69 years (median 46) with AML/MDS (12), ALL (6), lymphoma (7), MM (4), CLL (1) were conditioned with TBI or Melphalan, Thiothepa, and Fludarabine and given T alpha 1 (1.6 mg/day subcutaneously) from the day of transplant onwards for 16 weeks. Forty-five patients (25/45 with active disease at transplant), who were transplanted under the same protocol, served as controls. They were aged 20–67 years (median 53) with AML/MDS (20), ALL (4), lymphoma (11), MM (6), CLL (3), myelofibrosis (1). During and after T alfa 1 administration no adverse effects whatsoever were observed. Immune reconstitution was assessed by limiting dilution analyses of frequencies of CD4+ T cells that were specific for Aspergillus, Candida, CMV, Adenovirus, Herpes Simplex Virus, Varicella- Zoster Virus, Toxoplasma antigens. Normal donor values ranged from 600 to 1200/10e6 plated cells. Control transplant recipients acquired such pathogen-specific T cell responses from month 3 onwards in frequencies that ranged from 50 to 250/10e6 plated cells. In patients who received T alfa 1, pathogen-specific T cells appeared as early as 1 month after transplant in significantly higher frequencies which soon ranged from 250 to 500/10e6plated cells. The cumulative incidence of non-relapse mortality (NRM) (mainly infection-related) was 33% in controls vs 7% in Thymosin-treated patients (p = 0.02). Thymosin administration did not impact upon the relapse rate (which was around 50% in both series). As a consequence of the improved TRM, Event-Free Survival was better in Thymosin treated patients (42% vs 20% in controls; p = 0.02). Multivariate analyses including diagnoses, disease status at transplant, conditioning regimen and donor lymphocyte infusions (during and after Thymosin administration) showed Thymosin treatment was a significant independent factor predicting a lower incidence of NRM (p=0.04) which tended to provide better survival (p = 0.09). In conclusion, this study shows that T alfa 1, a naturally occurring peptide of thymic origin which optimizes antigen presentation and T cell responses, could safely be administered after matched sibling T cell depleted hematopoietic transplants. T alfa 1appeared to protect from largely infectious NRM and to improve the survival of transplant recipients. It is worth noting that T cell depletion of the graft and the consequent lack of post-transplant pharmacological immune suppression may have facilitated the peptide's immune regulatory action.