Graft T Cells Retard Stem Cell Derived Lymphoid Organ Reconstitution and Immune Function After Allogeneic Transplantation,

Abstract 4030

Impaired immune function is a major cause of morbidity and mortality after allogeneic hematopoietic cell transplantation (HCT). Adoptively transferred donor T cells (TC) mediate graft-versus-host disease (GVHD), and make the use of pharmacological immunosuppression necessary, yet are believed to protect against infections post-HCT (pTX). In an MHC-matched, minor antigen mismatched mouse model, we studied the impact of allogeneic doTC on GVH-related damage of lymphoid organs, their recovery and immune function pTX. Lethally irradiated BALB.B mice received purified hematopoietic stem cells (HSC; cKit⁺Sca1⁺Thy1.1^loLin⁻) +/− TC from C57BL/6 (B6) donors. Recipient lymphoid organs, marrow and livers were analyzed at multiple time points. Mice given HSC+TC promptly became full donor chimeras in all lineages and developed moderate GVHD. HSC recipients retained some residual host cells, and had no signs of GVHD. By tetramer analysis and ELISPOT assay, we showed that in pure HSC recipients early pTX residual host TC protected against murine cytomegalovirus infection. Once a nascent donor HSC-derived TC pool was established, these TC reacted robustly against the virus. In contrast, transferred mature allogeneic TC proliferated and expanded in HSC+TC recipients, but did not respond or protect against the virus. To better understand why MHC-matched mature donor TC did not react against this pathogen, even when derived from pre-immunized donors, we studied the lymphatic microenvironment, hypothesizing that only highly organized interactions between cellular and humoral immunity, antigen-presenting and innate immune cells provide full immune function. Comparing tissues of mice given pure HSC +/− TC we found that: (i) lethal radiation caused marked reduction in lymphoid organ cellularity at 2 wks pTX in all transplanted groups. While the cellularity increased rapidly in HSC recipients, severe hypocellularity persisted in HSC+TC recipients, was associated with significantly decreased organ size (lymph nodes [LN] and thymuses) and marked disruption of the histological architecture; (ii) TC phenotypes present in reconstituted mice comprised a mixture of naïve (CD62L⁺CD44⁻), central memory (CM; CD62L⁺CD44⁺) and effector memory (EM; CD62L⁻CD44⁺) cells in HSC recipients, but were largely EM and rarely naïve TC in HSC+TC recipients; (iii) at ∼3 wks pTX donor B cells (BC) dominated lymphoid tissues in HSC recipients, but were absent in HSC+ToTC recipients; (iv) NK cells regenerated promptly and steadily in HSC recipients, constituting up to 20% of all lymphoid cells in the liver at 3 wks pTX, while in HSC+TC recipients they incessantly decreased to near-complete absence; (v) in mesenteric (m) LN of HSC-, but not HSC+TC- recipients, CD4⁺CD8⁺CD3⁻ TC were observed, derived from donor HSC. This latter population resembled immature thymic TC, suggesting that peripheral lymphoid organs participate in lymphoid reconstitution in adult recipients; (vi) at 2 wks pTX we noted only in HSC, but not HSC+ToTC recipients persisting and regenerating innate lymphoid cells (ILC). No ‘classical' lymphoid tissue inducer cells, as described in embryogenesis of lymphoid organs, but cells with features of ILC were identified. Donor HSC-derived NKp46⁺RORgt⁺CD3⁻ NK-like cells and IL-17A-secreting residual host CD4 TC were present, especially in livers and mLN. Cells that secreted IL-17A only and IL-17A+IFNγ were identified. IL-17A was not detected in recipients of HSC+ToTC, rather their donor TC were characterized by high IFNγ levels. Our studies show that, paradoxically, pure HSC grafts have advantages over TC-replete grafts with regard to immune function pTX. We hypothesize this superiority may be due to the presence of ILC that participate in the repair of lymphoid organs, and the whole spectrum of immune cells, including TC, BC, innate and immature cells emerging in HSC recipients. An intact microenvironment appears to be critical for optimal activation of TC to respond to pathogens. Moreover, our studies imply that donor TC cannot exert their protective immune function, when rapidly expanding alloreactive TC prevent the development of other lymphoid and innate immune cell, and disrupt the lymphoid tissue architecture. Controlling the negative effects of mature donor TC remains a challenge, and requires better understanding of the complexity of functional immune reconstitution pTX.