Immune cell profiling in intestinal transplantation

Abstract

Since the first published case study of human intestinal transplantation in 1967, there have been significant studies of intestinal transplant immunology in both animal models and humans. An improved understanding of the profiles of different immune cell subsets is critical for understanding their contributions to graft outcomes. While different studies have focused on the contribution of one or a few subsets to intestinal transplant, no study has integrated these data for a comprehensive overview of immune dynamics after intestinal transplant. Here, we provide a systematic review of the literature on different immune subsets and discuss their roles in intestinal transplant outcomes on multiple levels, focusing on chimerism and graft immune reconstitution, clonal alloreactivity, and cell phenotype. In Sections 1, 2 and 3, we lay out a shared framework for understanding intestinal transplant, focusing on the mechanisms of rejection or tolerance in the context of mucosal immunology and illustrate the unique role of the bidirectional graft-versus-host (GvH) and host-versus-graft (HvG) alloresponse. In Sections 4, 5 and 6, we further expand upon these concepts as we discuss the contribution of different cell subsets to intestinal transplant. An improved understanding of intestinal transplantation immunology will bring us closer to maximizing the potential of this important treatment.

Keywords: Bidirectional alloresponse; Chimerism; Immune cell profiling; Intestinal transplantation.

Figure 1:. Key concepts in the bidirectional alloresponse after ITx.

(A) During quiescence, recipient circulating T cells infiltrate the intestinal graft mucosa and are activated by donor APCs into HvG-reactive Teff cells. (B) Recipient circulating APCs also simultaneously infiltrate the intestinal graft and activate donor T cells into GvH-reactive Teff cells. (C) During quiescence, the GvH response is greater than the HvG response locally. (D) GvH-reactive T cells migrate into recipient peripheral blood and bone marrow, where they mediate LGVHR to counteract recipient hematopoietic cells and HvG-reactive T cells, making space for donor graft-derived HSPCs engraft into the bone marrow to promote multilineage hematopoietic chimerism. (E) HvG-reactive Teff cells can undergo various differentiation pathways including towards Tfh, Th17, TRM, and Teff/TRM. Some pre-Tx MLR-defined HvG-reactive T cells become tolerant HvG cells that are dominated by TRM phenotype during quiescence. (F) Recipient T cells undergo dynamic exchange among the circulation, intestinal graft and native colon to gradually establish a stable TRM repertoire through the entire gastrointestinal tract. (G) Donor Tregs migrate into the circulation and partially contribute to the recipient-specific hyporesponsiveness of circulating donor T cells late post-Tx. (H) Circulating recipient B cells that enter the donor intestines can convert to BRMs or enter into lymphoid follicles to interact with Tfh cells. (I) During rejection, recipient circulating T cells enter the intestinal graft and are activated by donor APCs to locally expand into HvG-reactive Teff cells which can undergo various differentiation pathways. (J) The HvG response is greater than the GvH response and results in rejection. (K) HvG-reactive TRMs readopt Teff/TRM phenotypes and likely contribute to rejection. (L) Putative de novo H’vG T cell clones are predominantly Teff/TRM phenotypes and correlate with rejection and graft loss. (M) HvG-reactive T cells can enter into lymphoid follicles and obtain Tfh phenotypes to activate recipient B cells to produce donor specific antibodies (DSAs). (N) Tregs can adopt a Teff like phenotype during rejection. Created with BioRender.com. Red cells: donor origin, blue cells: recipient origin, gray cells: literature is not clear on donor/recipient origin or distinction does not apply.

Figure 2:. Adaptive immunity in ITx.

(A) During quiescence, intestinal graft mucosa CD103+ CD8 T cells express more polyfunctional cytokine whereas CD103− CD8 T cells express more cytotoxic markers. Recipient circulating APCs infiltrate the donor mucosa and stimulate GvH Teff cells to reduce the HvG response and facilitate bone marrow engraftment of donor HSPCs. A Teff/TRM to TRM differentiation trajectory among recipient T cells late post-Tx correlates with quiescent status. Circulating CD8 T cells express more markers of activation including HLA-DR. (B) During rejection, recipient CD8 TRMs readopt Teff/TRM phenotypes within intestinal graft. Activated circulating CD8 T cells infiltrate the lamina propria via α4β7. (C) During quiescence, Th1 cells with the most TRM like phenotype with expression of CD69 and CD103 also express the most Th1 type cytokine. (D) During rejection, both donor and recipient Th1 cells produce Th1 type cytokines as well as IP10 (CXCL10), recruiting additional Th1 cells, resulting in a positive feedback loop. (E) During quiescence, Th17 cells adopt a TRM like phenotype and produce IL22, IL17 and CCL20. IL17 is predominantly expressed by CD103+ rather than CD103− Th17 cells. (F) During rejection, CCR6+ Th17 cells secrete CCL20 which recruits additional Th17 cells via CCR6. (G) During quiescence, Tregs adopt a TRM like phenotype. Furthermore, donor circulating Tregs reduce the GvH response. Donor Tregs also migrate to the recipient bone marrow creating an immunosuppressive niche, likely facilitating infiltration and residence of donor HSPCs. (H) During rejection, recipient Tregs obtained Teff phenotype with acquired HvG reactivity. Furthermore, recipient Tregs have been shown to increase secretion of inflammatory Th1 and Th17 type cytokines. (I) During quiescence, recipient Tfh with tolerant feature prevent activation of recipient B cells. Donor-derived CD19− plasma cells were shown to persist for decades within the intestinal graft with a prosurvial phenotype. (J) During rejection, recipient B cells are activated by recipient Tfh cells (indirect response) or donor Tfh cells (inverted direct response) to stimulate production of DSAs. Created with BioRender.com. Red cells: donor origin, blue cells: recipient origin, gray cells: literature is not clear on donor/recipient origin or distinction does not apply, intestines: donor origin.

Figure 3:. Innate immunity in ITx.

(A) During quiescence, γδ T cells produce IL4, IL10, and TGFβ. In the mucosa, Vδ1 clonotypes predominate in contrast to Vδ2. Recipient γδ T cells exhibit faster repopulation dynamics than αβ T cells. There may be a role of donor γδ cells in mediating LGVHR. (B) During rejection, recipient γδ T cells likely undergo local expansion within intestinal graft mucosa and show an increased exchange between the blood and mucosa. (C) During quiescence, there is increased donor NCR+ ILC3 which produce IL22. Furthermore, recipient ILC1 and ILC3 repopulate the donor graft. Donor ILC3s, particularly CD56− ILC3s, tend to persist longer term in the intestinal graft compared to other types of donor classical ILCs. Recipient NK cells demonstrate increased repopulation dynamics compared to recipient T cells. Many NK cells persist long term in the graft and have been shown to express CD103. (D) During rejection, ILC1 cells predominate in the mucosa and express Th1 cytokines including IFNγ and TNFα. NCR− ILC3s are present in lower number. CD56hi NK cells are increased in circulation during rejection. (E) During quiescence, NKT cells produce IL4 and IL5 in the intestinal graft. MAIT cells are present in low frequency early after transplant but recover to normal levels around 1 year. (F) During rejection, the role of MAIT and NKT cells remains unclear. (G) During quiescence, recipient myeloid cell infiltrate donor graft. Recipient MDSCs have been shown to inhibit recipient T cell responses. Circulating MDSCs are increased during quiescence compared to rejection. Intestinal macrophages have four subsets (MF1, MF2, MF3, MF4). Inflammatory MF1 and MF2 phenotypes with expression of TNFα, IL1β, and IL6 were largely replaced early after transplant, while MF3 and MF4 with features of mature macrophages likely differentiated from MF1 and MF2 phenotypes were replaced at a significantly slower rate. (H) During rejection, myeloid cells promote inflammation via upregulation of TLRs. Furthermore, mutations in NOD2 causes dysfunction of CXCR31+ myeloid cells which normally support Paneth cell expression of anti-microbial peptides, allowing bacteria to infiltrate and initiate inflammation. Created with BioRender.com. Red cells: donor origin, blue cells: recipient origin, gray cells: literature is not clear on donor/recipient origin or distinction does not apply, intestines: donor origin.