Suxiao Yang

Tracking donor-reactive T cells: Evidence for clonal deletion in tolerant kidney transplant patients

Clonal deletion is a mechanism of graft tolerance after combined kidney and bone marrow transplantation in humans. Tolerating transplant Transplant rejection remains a formidable barrier to successful organ transplantation. Recent advances, such as combined kidney and bone marrow transplantation (CKBMT), hint that rejection can be overcome by the induction of immune tolerance. Now, Morris et al. have developed a way to track T cells to determine how this tolerance works. They used high-throughput T cell receptor sequencing to find donor-reactive T cells before transplant and then tracked these clones after CKBMT. These donor-reactive T cells were reduced in CKBMT patients who achieved tolerance but not in a CKBMT patient who failed to achieve tolerance or in recipients of conventional, nontolerizing transplant protocols. These data suggest that clonal deletion is a mechanism of graft tolerance after CKBMT in humans. T cell responses to allogeneic major histocompatibility complex antigens present a formidable barrier to organ transplantation, necessitating long-term immunosuppression to minimize rejection. Chronic rejection and drug-induced morbidities are major limitations that could be overcome by allograft tolerance induction. Tolerance was first intentionally induced in humans via combined kidney and bone marrow transplantation (CKBMT), but the mechanisms of tolerance in these patients are incompletely understood. We now establish an assay to identify donor-reactive T cells and test the role of deletion in tolerance after CKBMT. Using high-throughput sequencing of the T cell receptor B chain CDR3 region, we define a fingerprint of the donor-reactive T cell repertoire before transplantation and track those clones after transplant. We observed posttransplant reductions in donor-reactive T cell clones in three tolerant CKBMT patients; such reductions were not observed in a fourth, nontolerant, CKBMT patient or in two conventional kidney transplant recipients on standard immunosuppressive regimens. T cell repertoire turnover due to lymphocyte-depleting conditioning only partially accounted for the observed reductions in tolerant patients; in fact, conventional transplant recipients showed expansion of circulating donor-reactive clones, despite extensive repertoire turnover. Moreover, loss of donor-reactive T cell clones more closely associated with tolerance induction than in vitro functional assays. Our analysis supports clonal deletion as a mechanism of allograft tolerance in CKBMT patients. The results validate the contribution of donor-reactive T cell clones identified before transplant by our method, supporting further exploration as a potential biomarker of transplant outcomes.

Macrochimerism in Intestinal Transplantation: Association With Lower Rejection Rates and Multivisceral Transplants, Without GVHD.

Blood chimerism has been reported sporadically among visceral transplant recipients, mostly in association with graft‐vs‐host disease (GVHD). We hypothesized that a higher degree of mixed chimerism would be observed in multivisceral (MVTx) than in isolated intestinal (iITx) and isolated liver transplant (iLTx) recipients, regardless of GVHD. We performed a longitudinal prospective study investigating multilineage blood chimerism with flow cytometry in 5 iITx and 4 MVTx recipients up to one year posttransplant. Although only one iITx patient experienced GVHD, T cell mixed chimerism was detected in 8 out of 9 iITx/MVTx recipients. Chimerism was significantly lower in the four subjects who displayed early moderate to severe rejection. Pre‐formed high‐titer donor‐specific antibodies, bound in vivo to the circulating donor cells, were associated with an accelerated decline in chimerism. Blood chimerism was also studied in 10 iLTx controls. Among nonsensitized patients, MVTx recipients exhibited greater T and B cell chimerism than either iITx or iLTx recipients. Myeloid lineage chimerism was present exclusively among iLTx and MVTx (6/13) recipients, suggesting that its presence required the hepatic allograft. Our study demonstrates, for the first time, frequent T cell chimerism without GVHD following visceral transplantation and a possible relationship with reduced rejection rate in MVTx recipients.

Bidirectional intragraft alloreactivity drives the repopulation of human intestinal allografts and correlates with clinical outcome

Clinical transplant outcomes reflect the balance between host-versus-graft and graft-versus-host reactivities. Transplantation stalemate Successful organ transplantation depends on both preventing rejection of the graft by host cells [host versus graft (HvG)] and blocking graft cells that attack the host (GvH). Zuber et al. demonstrated that a balance in this two-way alloreactivity affected clinical outcomes in organ transplant recipients. They examined gut-resident T cell turnover kinetics in human intestinal allografts and found that HvG-reactive cells persisted long-term in the graft, acquiring a tissue-resident phenotype. However, GvH-reactive cells expanded within the graft in the absence of rejection. The GvH-reactive cells may balance out the HvG-reactive cells, preventing rejection. One paradigm in transplantation is that graft-infiltrating T cells are largely nonalloreactive “bystander” cells. However, the origin and specificity of allograft T cells over time have not been investigated in detail in animals or humans. We used polychromatic flow cytometry and high-throughput T cell receptor sequencing of serial biopsies to show that gut-resident T cell turnover kinetics in human intestinal allografts are correlated with the balance between intragraft host-versus-graft (HvG) and graft-versus-host (GvH) reactivities and with clinical outcomes. In the absence of rejection, donor T cells were enriched for GvH-reactive clones that persisted in the long term in the graft. Early expansion of GvH clones in the graft correlated with the rapid replacement of donor antigen-presenting cells by the recipient. Rejection was associated with transient infiltration by blood-like recipient CD28+ NKG2DHi CD8+ αβ T cells, marked predominance of HvG clones, and accelerated T cell turnover in the graft. Ultimately, these recipient T cells acquired a steady-state tissue-resident phenotype but regained CD28 expression during rejections. Increased ratios of GvH to HvG clones were seen in nonrejectors, potentially mitigating the constant threat of rejection posed by HvG clones persisting within the tissue-resident graft T cell population.

Long-term Persistence of Innate Lymphoid Cells in the Gut After Intestinal Transplantation

Background Little is known about innate lymphoid cell (ILC) populations in the human gut, and the turnover of these cells and their subsets after transplantation has not been described. Methods Intestinal samples were taken from 4 isolated intestine and 3 multivisceral transplant recipients at the time of any operative resection, such as stoma closure or revision. ILCs were isolated and analyzed by flow cytometry. The target population was defined as being negative for lineage markers and double-positive for CD45/CD127. Cells were further stained to define ILC subsets and a donor-specific or recipient-specific HLA marker to analyze chimerism. Results Donor-derived ILCs were found to persist greater than 8 years after transplantation. Additionally, the percentage of cells thought to be lymphoid tissue inducer cells among donor ILCs was far higher than that among recipient ILCs. Conclusions Our findings demonstrate that donor-derived ILCs persist long-term after transplantation and support the notion that human lymphoid tissue inducer cells may form in the fetus and persist throughout life, as hypothesized in rodents. Correlation between chimerism and rejection, graft failure, and patient survival requires further study.

Quantifying size and diversity of the human T cell alloresponse

Alloreactive T lymphocytes are the primary mediators of immune responses in transplantation, both in the graft-versus-host and host-versus-graft directions. While essentially all clones comprising the human T cell repertoire have been selected on self-peptide presented by self-human leukocyte antigens (self-HLAs), much remains to be understood about the nature of clones capable of responding to allo-HLA molecules. Quantitative tools to study these cells are critical to understand fundamental features of this important response; however, the large size and diversity of the alloreactive T cell repertoire in humans presents a great technical challenge. We have developed a high-throughput T cell receptor (TCR) sequencing approach to characterize the human alloresponse. We present a statistical method to model T cell clonal frequency distribution and quantify repertoire diversity. Using these approaches, we measured the diversity and frequency of distinct alloreactive CD4+ and CD8+ T cell populations in HLA-mismatched responder-stimulator pairs. Our findings indicate that the alloimmune repertoire is highly specific for a given pair of individuals, that most alloreactive clones circulate at low frequencies, and that a high proportion of TCRs is likely able to recognize alloantigens.

HSC extrinsic sex-related and intrinsic autoimmune disease–related human B-cell variation is recapitulated in humanized mice

B cells play a major role in antigen presentation and antibody production in the development of autoimmune diseases, and some of these diseases disproportionally occur in females. Moreover, immune responses tend to be stronger in female vs male humans and mice. Because it is challenging to distinguish intrinsic from extrinsic influences on human immune responses, we used a personalized immune (PI) humanized mouse model, in which immune systems were generated de novo from adult human hematopoietic stem cells (HSCs) in immunodeficient mice. We assessed the effect of recipient sex and of donor autoimmune diseases (type 1 diabetes [T1D] and rheumatoid arthritis [RA]) on human B-cell development in PI mice. We observed that human B-cell levels were increased in female recipients regardless of the source of human HSCs or the strain of immunodeficient recipient mice. Moreover, mice injected with T1D- or RA-derived HSCs displayed B-cell abnormalities compared with healthy control HSC-derived mice, including altered B-cell levels, increased proportions of mature B cells and reduced CD19 expression. Our study revealed an HSC-extrinsic effect of recipient sex on human B-cell reconstitution. Moreover, the PI humanized mouse model revealed HSC-intrinsic defects in central B-cell tolerance that recapitulated those in patients with autoimmune diseases. These results demonstrate the utility of humanized mouse models as a tool to better understand human immune cell development and regulation.