Graft-vs-host disease (GVHD) is a common complication of allogeneic stem cell transplant (alloSCT) wherein donor T cells target alloantigens on recipient tissues. It is unclear how alloimmune responses are maintained in GVHD despite abundant antigen, which causes T cell anergy, deletion and exhaustion. Previously, we identified alloreactive TCF-1 high T cells arising post-transplant that resemble exhausted progenitors (T EXP) capable of propagating immune responses in other chronic antigen models. Here, we sought to further characterize these cells in the B6→129 MHC-matched GVHD mouse model, in which 129 recipients express the immunodominant H-2K b-restricted minor histocompatibility antigen (miHA) H60. At day +7 post-transplant, alloreactive CD8 + cells specific to H60 (as determined by MHC-I-tetramer staining; Tet H60+) were nearly uniformly PD-1 hiTox hi whereas Tet H60- cells displayed a bimodal distribution into discrete PD-1 hiTox hi and PD-1 loTox lo populations, indicative of more diverse antigen experiences. Among these both Tet H60+ and Tet H60- cells were TCF-1 hi cells. TCF-1 hi Tet H60+ cells were uniformly CD39 loTox hiPD-1 hi, which is a canonical T EXP phenotype. In contrast, among activated Tet H60- cells there were TCF-1 hi cells that were CD39 loTox hiPD-1 hi and Tox loPD-1 lo. At later times in spleen and lymph node, and in GVHD target tissues, these populations of TCF-1 + Tet H60+ and Tet H60- were found. To test if these CD39 loTCF-1 hi T EXP had proliferative advantages in GVHD, we sorted congenic TCF-1 hiCD39 lo and TCF-1 loCD39 hi CD8 + cells from recipient spleens 14-days post-transplant and adoptively transferred them in competition in a 1:1 ratio (of Tet H60+ cells) into newly transplanted recipients. Among Tet H60+ cells in all tissues at day 14 post-transfer, TCF-1 hiCD39 lo-sorted progeny greatly outperformed TCF-1 loCD39 hi-sorted progeny. In line with their role as a source of GVHD effectors, progeny of TCF-1 hiCD39 lo cells were mostly TCF-1 loCD39 hi; however, a fraction remained TCF-1 hi consistent with their being able to undergo self-renewal. Conversely, we observed few if any TCF-1 + progeny of CD39 hi cells. We next tested whether TCF-1 was an important mediator of T cell fitness or whether it was only a marker for functionality. To do so we competed congenic wild-type (WT) and Tcf7 p45-/- (p45 -/-) donor CD8 cells, which lack the N-terminal β-catenin binding domain of TCF-1, in allogeneic (129) and syngeneic (B6) recipients. Strikingly, p45 -/- CD8 cells were greatly outcompeted by WT CD8 cells in 129 recipients in all tissues and at all times post-transplant, among both Tet H60+ and Tet H60- cells. In contrast, in B6 recipients, WT and p45 -/- cells remained evenly matched, suggesting that full-length TCF-1 isoforms are dispensable for lymphopenia-induced T cell expansion. Further, p45 -/- cells were also not disadvantaged when adoptively transferred into B6 mice and acutely challenged with H60 antigen by vaccination. Together these data suggest a model wherein TCF-1 hi progenitor like T cells are seeded in GVHD target organs where they may serve as a key local source for GVHD effectors, and moreover, full-length TCF-1 is itself critical for alloreactive T cell fitness in GVH responses.

Graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation is a major cause of nonrelapse morbidity and mortality. Although ruxolitinib is now approved for the treatment of steroidrefractory GVHD, to date, no agent added to corticosteroids has been shown to improve outcomes compared with corticosteroids alone. In this issue of Blood Advances, Al Malki et al presented the results of a multicenter phase 2 study that tested whether the addition of natalizumab, a humanized antibody against the α4 subunit of α4β7 integrin, would improve the outcomes of new-onset acute GVHD. The primary end point was a complete response after 28 days, defined as the clinical resolution of GVHD in the target organs.

Recipient effector T cells differentiate into functional tissue-resident memory T cells, causing graft rejection after kidney transplantation. Memories of rejection Long-term graft survival after organ transplantation can be hindered by immune-mediated allograft rejection; thus, understanding these immune responses is crucial to developing new transplant-supporting therapies. Tissue-resident memory T cells (TRM), a subset of memory T cells that reside in barrier tissues and do not recirculate, are detectable in transplanted organs, but it is unclear if they contribute to allograft rejection. Abou-Daya et al. created a mouse model of T cell–mediated kidney transplant rejection, showing that adoptively transferred, kidney antigen–specific effector T cells differentiated into functional, nonrecirculating antigen-specific TRM in the transplanted kidneys. These kidney antigen–specific TRM induced allograft rejection. These data suggest that TRM in transplanted allografts can contribute to rejection and that targeting alloreactive TRM might improve long-term graft survival in transplant recipients. Tissue-resident memory T cells (TRM) contained at sites of previous infection provide local protection against reinfection. Whether they form and function in organ transplants where cognate antigen persists is unclear. This is a key question in transplantation as T cells are detected long term in allografts, but it is not known whether they are exhausted or are functional memory T cells. Using a mouse model of kidney transplantation, we showed that antigen-specific and polyclonal effector T cells differentiated in the graft into TRM and subsequently caused allograft rejection. TRM identity was established by surface phenotype, transcriptional profile, and inability to recirculate in parabiosis and retransplantation experiments. Graft TRM proliferated locally, produced interferon-γ upon restimulation, and their in vivo depletion attenuated rejection. The vast majority of antigen-specific and polyclonal TRM lacked phenotypic and transcriptional exhaustion markers. Single-cell analysis of graft T cells early and late after transplantation identified a transcriptional program associated with transition to the tissue-resident state that could serve as a platform for the discovery of therapeutic targets. Thus, recipient effector T cells differentiate into functional graft TRM that maintain rejection locally. Targeting these TRM could improve renal transplant outcomes.

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in allogeneic hematopoietic stem cell transplantation (alloSCT). In GVHD, donor T cells recognize recipient tissues as non-self and mount a broad attack that results in multi-organ damage. While there has been some success in diminishing the incidence of GVHD, less progress has been made in treating established or steroid-refractory disease without severe global immunosuppression. This is in part due to a lack of understanding of the underlining mechanisms that sustain GVHD despite chronic T cell antigen exposure. To address this, we developed a mouse GVHD model that allows us to track the progeny of single alloreactive T cell clones. We used this model to test hypotheses on GVHD maintenance: 1) GVHD is driven by the continuous output and trafficking of alloreactive T cells from secondary lymphoid tissues (SLT) into GVHD target organs; and 2) once tissues are seeded with alloreactive T cells from SLT, GVHD is maintained locally within affected tissues. We reasoned that if GVHD is maintained by the continuous SLT output of T cells, the progeny of single alloreactive clones in a target tissue would come into equilibrium with those in SLT and other target tissues. Alternatively, if there is a degree of local tissue GVHD maintenance, our model predicts that clonal progeny should be unequally distributed and not in equilibrium with SLT. We first tested these possibilities using a GVHD model wherein BALB/c RAG2-/- TCR transgenic (Tg) CD4 T cells (TS1) that recognize the S1 peptide derived from influenza hemagglutinin (HA) induce GVHD in BALB/c RAG2-/- mice that ubiquitously express HA (HA104 mice). We generated TS1 TCR Tg mice on 9 congenic backgrounds based on the expression of CD45.1/2, Thy1.1/2 and GFP. We transferred 500 naïve TS1 cells of 1 clonotype (to induce GVHD) along with single naïve TS1 cells from the remaining 8 clonotypes and BALB/c RAG2-/- bone marrow (BM) into lethally irradiated HA104 mice. We recovered a total of 432 single-cell derived TS1 clones (72% of input clones) from tissues of 79 mice, analyzed 7-35 days after transfer. We enumerated the TS1 clonal composition of each tissue (expressed as the % of all TS1 of that tissue) and found disparate clonal distribution across tissues within individual mice. For example, in a representative mouse analyzed at day 33 (Figure 1), the fractions of a single TS1 clone were relatively high in the colon (1.4%) and small intestine intraepithelial lymphocyte (IEL) (4.6%) when compared to the spleen (0.07%), BM (0.02%) and liver (0.03%). These data support that TS1 clones are not equally distributed among tissues and are not in equilibrium with SLT, suggesting that GVHD is at least in part maintained locally. We also analyzed TS1 clonal frequency distribution in a second model. BALB/c RAG2-/- BM and polyclonal T cells were transplanted into F1 (BALB/c HA104xB10.D2) recipients along with 8 single distinct TS1 cells. In this system, GVHD is induced by polyclonal BALB/c cells and TS1 cells are trackers of reactivity to HA. Preliminary experiments also indicate unequal distributions of TS1 clones across tissues in individual mice. In a second approach to test whether GVHD is maintained locally, irradiated HA104 mice were reconstituted with either 500 Thy1.1 or 500 Thy1.2 TS1 cells and CD45.1 or CD45.2 BALB/c RAG2-/- BM. One partner also received congenic TS1 single cells. We performed parabiosis of mice from one group to the other 21-28 days later. We analyzed 4 pairs 4 weeks post-joining, looking first at the blood to establish a baseline for TS1 crossover from the parabiotic partner. In blood, 18.9% ±1.9 of all TS1 were derived from the partner. Importantly, relative to equilibration in blood, there were far fewer partner-derived TS1 cells in all other tissues (Figure 2). Only a few single cell-derived TS1 clones were detected in the partner mouse at very low frequencies, even when they were dominant in tissues of the corresponding partner. Together, these data indicate that once GVHD is established, local maintenance dominates over new TS1 entry. Consistent with this, TS1 cells incorporate BrdU in vivo even at late time points. We are combining proliferation and clonality data at multiple timepoints to develop a mathematical model of GVHD establishment and maintenance. We are also extending our observations in a polyclonal GVHD model wherein the progeny from single alloreactive CD8 cells can be enumerated. Shlomchik: NapaJen: Consultancy.

Effector memory T cells (TEM) are less capable of inducing graft-versus-host disease (GVHD) compared with naive T cells (TN). Previously, in the TS1 TCR transgenic model of GVHD, wherein TS1 CD4 cells specific for a model minor histocompatibility Ag (miHA) induce GVHD in miHA-positive recipients, we found that cell-intrinsic properties of TS1 TEM reduced their GVHD potency relative to TS1 TN. Posttransplant, TS1 TEM progeny expressed higher levels of PD-1 than did TS1 TN progeny, leading us to test the hypothesis that TEM induce less GVHD because of increased sensitivity to PD-ligands. In this study, we tested this hypothesis and found that indeed TS1 TEM induced more severe skin and liver GVHD in the absence of PD-ligands. However, lack of PD-ligands did not result in early weight loss and colon GVHD comparable to that induced by TS1 TN, indicating that additional pathways restrain alloreactive TEM. TS1 TN also caused more severe GVHD without PD-ligands. The absence of PD-ligands on donor bone marrow was sufficient to augment GVHD caused by either TEM or TN, indicating that donor PD-ligand–expressing APCs critically regulate GVHD. In the absence of PD-ligands, both TS1 TEM and TN induced late-onset myocarditis. Surprisingly, this was an autoimmune manifestation, because its development required non-TS1 polyclonal CD8+ T cells. Myocarditis development also required donor bone marrow to be PD-ligand deficient, demonstrating the importance of donor APC regulatory function. In summary, PD-ligands suppress both miHA-directed GVHD and the development of alloimmunity-induced autoimmunity after allogeneic hematopoietic transplantation.

Erythroid enucleation is the process by which the future red blood cell disposes of its nucleus prior to entering the blood stream. This key event during red blood cell development has been likened to an asymmetric cell division (ACD), by which the enucleating erythroblast divides into two very different daughter cells of alternate molecular composition, a nucleated cell that will be removed by associated macrophages, and the reticulocyte that will mature to the definitive erythrocyte. Here we investigated gene expression of members of the Par, Scribble and Pins/Gpsm2 asymmetric cell division complexes in erythroid cells, and functionally tested their role in erythroid enucleation in vivo and ex vivo. Despite their roles in regulating ACD in other contexts, we found that these polarity regulators are not essential for erythroid enucleation, nor for erythroid development in vivo. Together our results put into question a role for cell polarity and asymmetric cell division in erythroid enucleation.