The Role of Lymphoid Neogenesis in Allografts

iPSCs engrafted in allogeneic hosts without immunosuppression induce donor-specific tolerance to secondary allografts

Currently, most cell or tissue transplantations using induced pluripotent stem cells (iPSCs) are anticipated to involve allogeneic iPSCs. However, the immunological properties of iPSCs in an allogeneic setting are not well understood. We previously established a mouse transplantation model of MHC-compatible/minor antigen-mismatched combinations, assuming a hypoimmunogenic iPSC-setting. Here, we found that iPSCs subcutaneously inoculated into MHC-compatible allogeneic host mice resisted rejection and formed teratomas without immunosuppressant administration. Notably, when skin grafts were transplanted onto hosts more than 40 d after the initial iPSCs inoculation, only the skin of the same strain as the initial iPSCs was engrafted. Therefore, donor-specific immune tolerance was induced by a single iPSC inoculation. Diverse analyses, including single-cell RNA-sequencing after transplantation, revealed an increase in regulatory T cell (Treg) population, particularly CD25+ CD103+ effector Tregs within the teratoma and skin grafts. The removal of CD25+ or Foxp3+ cells suppressed the increase in effector Tregs and disrupted graft acceptance, indicating the importance of these cells in the establishment of immune tolerance. Within the teratoma, we observed an increase in TGF-β2 levels, suggesting an association with the increase in effector Tregs. Our results provide important insights for future applications of allogeneic iPSC-based cell or tissue transplantation.

Cardiac allograft tolerance can be achieved in nonhuman primates by donor bone marrow and kidney cotransplantation

Long-term, immunosuppression-free allograft survival has been induced in human and nonhuman primate (NHP) kidney recipients after nonmyeloablative conditioning and donor bone marrow transplantation (DBMT), resulting in transient mixed hematopoietic chimerism. However, the same strategy has consistently failed in NHP heart transplant recipients. Here, we investigated whether long-term heart allograft survival could be achieved by cotransplanting kidneys from the same donor. Cynomolgus monkeys were transplanted with heart allografts alone or heart and kidney allografts from the same major histocompatibility complex (MHC)-mismatched donors. All animals except one received DBMT, either at the same time or after a 2- to 4-month delay, plus short-term costimulation blockade and calcineurin inhibitor treatment. Long-term, immunosuppression-free heart allograft survival was consistently achieved in heart/kidney, but not heart-alone, recipients. This was not associated with greater donor/recipient histocompatibility or altered lymphoid cell reconstitution after conditioning. The maintenance of tolerance after heart/kidney transplantation was associated with the presence of forkhead box P3 (Foxp3+) regulatory T cell (Treg)-rich organized lymphoid structures in kidneys but not hearts. Substituting high-dose erythropoietin treatment for kidney transplantation was unsuccessful, suggesting that it was not the sole mechanism of action. RNA sequencing analysis revealed that gene expression in hearts from tolerant recipients closely resembled that in hearts from chronically immunosuppressed recipients but differed markedly from rejecting allografts and naïve hearts. A version of this protocol may be able to induce tolerance in patients with end-stage heart and kidney failure who require combined heart and kidney transplantation.

Local intragraft humoral immune responses in chronic lung allograft dysfunction

BACKGROUND

Donor human leukocyte antigen (HLA)-specific antibodies (DSA) and non-HLA antibodies can cause allograft injury, possibly leading to chronic lung allograft dysfunction (CLAD) after lung transplantation. It remains unclear whether these antibodies are produced locally in the graft or derived solely from circulation. We hypothesized that DSA and non-HLA antibodies are produced in CLAD lungs.

METHODS

Lung tissue was prospectively collected from 15 CLAD patients undergoing retransplantation or autopsy. 0.3 g of fresh lung tissue was cultured for 4 days without or with lipopolysaccharide or CD40L: lung culture supernatant (LCS) was sampled. Protein eluate was obtained from 0.3 g of frozen lung tissue. The mean fluorescence intensity (MFI) of DSA and non-HLA antibodies was measured by Luminex and antigen microarray, respectively.

RESULTS

LCS from all 4 patients who had serum DSA at lung isolation were positive for DSA, with higher levels measured after CD40L stimulation (CD40L+LCS). Of these, only 2 had detectable DSA in lung eluate. MFI of non-HLA antibodies from CD40L+LCS correlated with those from lung eluate but not with those from sera. Flow cytometry showed higher frequencies of activated lung B cells in patients whose CD40L+LCS was positive for DSA (n = 4) or high non-HLA antibodies (n = 6) compared to those with low local antibodies (n = 5). Immunofluorescence staining showed CLAD lung lymphoid aggregates with local antibodies contained larger numbers of IgG+ plasma cells and greater IL-21 expression.

CONCLUSIONS

We show that DSA and non-HLA antibodies can be produced within activated B cell-rich lung allografts.

Spectrum of chronic lung allograft dysfunction pathology in human lung transplantation

BACKGROUND

Long-term survival after lung transplantation (LTx) remains limited by chronic lung allograft dysfunction (CLAD), which includes 2 main phenotypes: bronchiolitis obliterans syndrome (BOS) and restrictive allograft syndrome (RAS), with possible overlap. We aimed to detail and quantify pathological features of these CLAD sub-types.

METHODS

Peripheral and central paraffin-embedded explanted lung samples were obtained from 20 consecutive patients undergoing a second LTx for CLAD, from 3 lobes. Thirteen lung samples, collected from non-transplant lobectomies or donor lungs, were used as controls. Blinded semi-quantitative grading was performed to assess airway fibrotic changes, parenchymal and pleural fibrosis, and epithelial and vascular abnormalities.

RESULTS

CLAD lung samples had higher scores for all airway- and lung-related parameters compared to controls. There was a notable overlap in histologic scores between BOS and RAS, with a wide range of scores in both conditions. Parenchymal and vascular fibrosis scores were significantly higher in RAS compared to BOS (p = 0.003 for both). We observed a significant positive correlation between the degree of inflammation around each airway, the severity of epithelial changes, and airway fibrosis. Immunofluorescence staining demonstrated a trend toward a lower frequency of club cells in CLAD and a higher frequency of apoptotic club cells in BOS samples (p = 0.01).

CONCLUSIONS

CLAD is a spectrum of airway, parenchymal, and pleural fibrosis, as well as epithelial, vascular, and inflammatory pathologic changes, where BOS and RAS overlap significantly. Our semi-quantitative grading score showed a generally high inter-reader reliability and may be useful for future CLAD histologic assessments.

Early graft-infiltrating lymphocytes are not associated with graft rejection in a mouse model of skin transplantation

Graft-infiltrating lymphocytes (GILs) play an important role in promoting rejection after organ transplantation. We recently reported that GILs that accumulated up to 3 days post-transplantation did not promote rejection, whereas GILs present 3-5 days post-transplantation promoted rejection in a mouse heart transplantation model. However, the immunological behaviour of GILs in murine skin transplantation remains unclear. GILs were isolated on days 3, 5 or 7 post-transplantation from C57BL/6 (B6) allogeneic skin grafts transplanted onto BALB/c mice. BALB/c Rag2-/- γc-/- mice (BRGs) underwent B6 skin graft transplantation 10 weeks after adoptive transfer of day 3, 5, or 7 GILs. BRGs reconstituted with day 5 or 7 GILs completely rejected B6 grafts. However, when B6 grafts harvested from recipient BALB/c mice on day 5 or 7 were re-transplanted into BRGs, half of the re-transplanted day 5 grafts established long-term survival, although all re-transplanted day 7 grafts were rejected. BRGs reconstituted with day 3 GILs did not reject B6 grafts. Consistently, re-transplantation using day 3 skin grafts resulted in no rejection. Administration of anti-CD25 antibodies did not prevent the phenomenon observed for the day 3 skin grafts. Furthermore, BRGs reconstituted with splenocytes from naïve BALB/c mice immediately rejected the naïve B6 skin grafts and the re-transplanted day 3 B6 grafts, suggesting that day 3 GILs were unable to induce allograft rejection during the rejection process. In conclusion, the immunological role of GILs depends on the time since transplantation. Day 3 GILs had neither protective nor alloreactive effects in the skin transplant model.

Tertiary lymphoid structures in diseases: immune mechanisms and therapeutic advances

Tertiary lymphoid structures (TLSs) are defined as lymphoid aggregates formed in non-hematopoietic organs under pathological conditions. Similar to secondary lymphoid organs (SLOs), the formation of TLSs relies on the interaction between lymphoid tissue inducer (LTi) cells and lymphoid tissue organizer (LTo) cells, involving multiple cytokines. Heterogeneity is a distinguishing feature of TLSs, which may lead to differences in their functions. Growing evidence suggests that TLSs are associated with various diseases, such as cancers, autoimmune diseases, transplant rejection, chronic inflammation, infection, and even ageing. However, the detailed mechanisms behind these clinical associations are not yet fully understood. The mechanisms by which TLS maturation and localization affect immune function are also unclear. Therefore, it is necessary to enhance the understanding of TLS development and function at the cellular and molecular level, which may allow us to utilize them to improve the immune microenvironment. In this review, we delve into the composition, formation mechanism, associations with diseases, and potential therapeutic applications of TLSs. Furthermore, we discuss the therapeutic implications of TLSs, such as their role as markers of therapeutic response and prognosis. Finally, we summarize various methods for detecting and targeting TLSs. Overall, we provide a comprehensive understanding of TLSs and aim to develop more effective therapeutic strategies.

What makes the kidney so tolerant?

Various organ allografts differ in their propensity to be spontaneously accepted without any immunosuppressive treatment. Understanding the mechanisms behind these differences can aid in managing alloimmune responses in general. C57BL/6 mice naturally accept DBA/2J kidney allografts, forming tertiary lymphoid organs containing regulatory T cells (rTLOs), crucial for graft acceptance. In this issue of the JCI, Yokose and colleagues revealed that rTLOs promote conversion of cytotoxic alloreactive CD8+ T cells into exhausted/regulatory ones, through an IFN-γ-mediated mechanism. Their study provides insights into tolerance development that could help promote the acceptance of grafts at higher risk of rejection.

Dysfunction of infiltrating cytotoxic CD8+ T cells within the graft promotes murine kidney allotransplant tolerance

Tolerance of mouse kidney allografts arises in grafts that develop regulatory tertiary lymphoid organs (rTLOs). Single-cell RNA-seq (scRNA-seq) data and adoptive transfer of alloreactive T cells after transplantation showed that cytotoxic CD8+ T cells are reprogrammed within the accepted graft to an exhausted/regulatory-like phenotype mediated by IFN-γ. Establishment of rTLOs was required because adoptive transfer of alloreactive T cells prior to transplantation results in kidney allograft rejection. Despite the presence of intragraft CD8+ cells with a regulatory phenotype, they were not essential for the induction and maintenance of kidney allograft tolerance since renal allotransplantation into CD8-KO recipients resulted in acceptance and not rejection. Analysis of scRNA-seq data from allograft kidneys and malignant tumors identified similar regulatory-like cell types within the T cell clusters and trajectory analysis showed that cytotoxic CD8+ T cells are reprogrammed into an exhausted/regulatory-like phenotype intratumorally. Induction of cytotoxic CD8+ T cell dysfunction of infiltrating cells appears to be a beneficial mechanistic pathway that protects the kidney allotransplant from rejection through a process we call "defensive tolerance." This pathway has implications for our understanding of allotransplant tolerance and tumor resistance to host immunity.

Report of the 2022 Banff Heart Concurrent: Focus on non-human leukocyte antigen antibodies in rejection and the pathology of "mixed" rejection

The Banff Heart Concurrent Session, held as part of the 16th Banff Foundation for Allograft Pathology Conference at Banff, Alberta, Canada, on September 21, 2022, focused on 2 major topics: non-human leukocyte antigen (HLA) antibodies and mixed rejection. Each topic was addressed in a multidisciplinary fashion with clinical, immunological, and pathology perspectives and future developments and prospectives. Following the Banff organization model and principles, the collective aim of the speakers on each topic was to • Determine current knowledge gaps in heart transplant pathology • Identify limitations of current pathology classification systems • Discuss next steps in addressing gaps and refining classification system.

IL-21-producing effector Tfh cells promote B cell alloimmunity in lymph nodes and kidney allografts

Follicular helper T (Tfh) cells have been implicated in controlling rejection after allogeneic kidney transplantation, but the precise subsets, origins, and functions of Tfh cells in this process have not been fully characterized. Here we show that a subset of effector Tfh cells marked by previous IL-21 production is potently induced during allogeneic kidney transplantation and is inhibited by immunosuppressive agents. Single-cell RNA-Seq revealed that these lymph node (LN) effector Tfh cells have transcriptional and clonal overlap with IL-21-producing kidney-infiltrating Tfh cells, implicating common origins and developmental trajectories. To investigate the precise functions of IL-21-producing effector Tfh cells in LNs and allografts, we used a mouse model to selectively eliminate these cells and assessed allogeneic B cell clonal dynamics using a single B cell culture system. We found that IL-21-producing effector Tfh cells were essential for transplant rejection by regulating donor-specific germinal center B cell clonal dynamics both systemically in the draining LN and locally within kidney grafts. Thus, IL-21-producing effector Tfh cells have multifaceted roles in Ab-mediated rejection after kidney transplantation by promoting B cell alloimmunity.

Intragraft B cell differentiation during the development of tolerance to kidney allografts is associated with a regulatory B cell signature revealed by single cell transcriptomics

Mouse kidney allografts are spontaneously accepted in select, fully mismatched donor-recipient strain combinations, like DBA/2J to C57BL/6 (B6), by natural tolerance. We previously showed accepted renal grafts form aggregates containing various immune cells within 2 weeks posttransplant, referred to as regulatory T cell-rich organized lymphoid structures, which are a novel regulatory tertiary lymphoid organ. To characterize the cells within T cell-rich organized lymphoid structures, we performed single-cell RNA sequencing on CD45+ sorted cells from accepted and rejected renal grafts from 1-week to 6-months posttransplant. Analysis of single-cell RNA sequencing data revealed a shifting from a T cell-dominant to a B cell-rich population by 6 months with an increased regulatory B cell signature. Furthermore, B cells were a greater proportion of the early infiltrating cells in accepted vs rejecting grafts. Flow cytometry of B cells at 20 weeks posttransplant revealed T cell, immunoglobulin domain and mucin domain-1+ B cells, potentially implicating a regulatory role in the maintenance of allograft tolerance. Lastly, B cell trajectory analysis revealed intragraft differentiation from precursor B cells to memory B cells in accepted allografts. In summary, we show a shifting T cell- to B cell-rich environment and a differential cellular pattern among accepted vs rejecting kidney allografts, possibly implicating B cells in the maintenance of kidney allograft acceptance.

Transcriptionally Distinct B Cells Infiltrate Allografts After Kidney Transplantation

BACKGROUND

Following allogeneic kidney transplantation, a substantial proportion of graft loss is attributed to the formation of donor-specific antibodies and antibody-mediated rejection. B cells infiltrate kidney grafts during antibody-mediated rejection; however, the origins, repertoires, and functions of these intrarenal B cells remain elusive.

METHODS

Here, we use murine allogeneic kidney transplant models to study the origins, transcriptional programming and B cell receptor repertoire of intragraft B cells, and in vitro stimulation assays to evaluate the ability of intragraft B cells to promote CD4+ T cell expansion.

RESULTS

B cells infiltrate kidney grafts in settings of allogeneic, but not syngeneic, transplantation. Intragraft B cells have characteristics of activation but are transcriptionally distinct from germinal center B cells and resemble innate-like B cells. B cell receptor sequencing demonstrates that the majority of intragraft B cells do not originate from lymph node germinal center B cells and are largely germline. Class-switched intragraft B cells are rare but can be donor-specific and produce IgG capable of binding to the kidney allograft. Lastly, intrarenal B cells are capable of stimulating naive T cells but have an altered ability to promote T follicular helper cell expansion.

CONCLUSIONS

Together, these data demonstrate that intrarenal B cells during transplant rejection are transcriptionally distinct from lymph node B cells.

Ferret Lung Transplantation Models Differential Lymphoid Aggregate Morphology Between Restrictive and Obstructive Forms of Chronic Lung Allograft Dysfunction

BACKGROUND

Long-term survival after lung transplantation remains limited by chronic lung allograft dysfunction (CLAD). CLAD has 2 histologic phenotypes, namely obliterative bronchiolitis (OB) and restrictive alveolar fibroelastosis (AFE), which have distinct clinical presentations, pathologies, and outcomes. Understanding of OB versus AFE pathogenesis would improve with better animal models.

METHODS

We utilized a ferret orthotopic single-lung transplantation model to characterize allograft fibrosis as a histologic measure of CLAD. Native lobes and "No CLAD" allografts lacking aberrant histology were used as controls. We used morphometric analysis to evaluate the size and abundance of B-cell aggregates and tertiary lymphoid organs (TLOs) and their cell composition. Quantitative RNA expression of 47 target genes was performed simultaneously using a custom QuantiGene Plex Assay.

RESULTS

Ferret lung allografts develop the full spectrum of human CLAD histology including OB and AFE subtypes. While both OB and AFE allografts developed TLOs, TLO size and number were greater with AFE histology. More activated germinal center cells marked by B-cell lymphoma 6 Transcription Repressor, (B-cell lymphoma 6) expression and fewer cells expressing forkhead box P3 correlated with AFE, congruent with greater diffuse immunoglobulin, plasma cell abundance, and complement 4d staining. Furthermore, forkhead box P3 RNA induction was significant in OB allografts specifically. RNA expression changes were seen in native lobes of animals with AFE but not OB when compared with No CLAD native lobes.

CONCLUSIONS

The orthotopic ferret single-lung transplant model provides unique opportunities to better understand factors that dispose allografts to OB versus AFE. This will help develop potential immunomodulatory therapies and antifibrotic approaches for lung transplant patients.

B Cell Immunity in Lung Transplant Rejection - Effector Mechanisms and Therapeutic Implications

Allograft rejection remains the major hurdle in lung transplantation despite modern immunosuppressive treatment. As part of the alloreactive process, B cells are increasingly recognized as modulators of alloimmunity and initiators of a donor-specific humoral response. In chronically rejected lung allografts, B cells contribute to the formation of tertiary lymphoid structures and promote local alloimmune responses. However, B cells are functionally heterogeneous and some B cell subsets may promote alloimmune tolerance. In this review, we describe the current understanding of B-cell-dependent mechanisms in pulmonary allograft rejection and highlight promising future strategies that employ B cell-targeted therapies.

Novel intragraft regulatory lymphoid structures in kidney allograft tolerance

Intragraft events thought to be relevant to the development of tolerance are here subjected to a comprehensive mechanistic study during long-term spontaneous tolerance that occurs in C57BL/6 mice that receive life sustaining DBA/2 kidneys. These allografts rapidly develop periarterial Treg-rich organized lymphoid structures (TOLS) that form in response to class II but not to class I MHC disparity and form independently of lymphotoxin α and lymphotoxin β receptor pathways. TOLS form in situ in the absence of lymph nodes, spleen, and thymus. Distinctive transcript patterns are maintained over time in TOLS including transcripts associated with Treg differentiation, T cell checkpoint signaling, and Th2 differentiation. Pathway transcripts related to inflammation are expressed in early stages of accepted grafts but diminish with time, while B cell transcripts increase. Intragraft transcript patterns at one week posttransplant distinguish those from kidneys destined to be rejected, that is, C57BL/6 allografts into DBA/2 recipients, from those that will be accepted. In contrast to inflammatory tertiary lymphoid organs (iTLOs) that form in response to chronic viral infection and transgenic Lta expression, TOLS lack high endothelial venules and germinal centers. TOLS represent a novel, pathogenetically important type of TLO that are in situ markers of regulatory tolerance.

Advanced Tertiary Lymphoid Tissues in Protocol Biopsies are Associated with Progressive Graft Dysfunction in Kidney Transplant Recipients

BACKGROUND

Tertiary lymphoid tissues (TLTs) are ectopic lymphoid tissues found in chronically inflamed organs. Although studies have documented TLT formation in transplanted kidneys, the clinical relevance of these TLTs remains controversial. We examined the effects of TLTs on future graft function using our histologic TLT maturity stages and the association between TLTs and Banff pathologic scores. We also analyzed the risk factors for the development of TLTs.

METHODS

Serial protocol biopsy samples (0 hour, 1, 6, and 12 months) without rejection were retrospectively analyzed from 214 patients who underwent living donor kidney transplantation. TLTs were defined as lymphocyte aggregates with signs of proliferation and their stages were determined by the absence (stage I) or presence (stage II) of follicular dendritic cells.

RESULTS

Only 4% of patients exhibited TLTs at the 0-hour biopsy. Prevalence increased to almost 50% at the 1-month biopsy, and then slightly further for 12 months. The proportion of advanced stage II TLTs increased gradually, reaching 19% at the 12-month biopsy. Presence of stage II TLTs was associated with higher risk of renal function decline after transplantation compared with patients with no TLT or stage I TLTs. Stage II TLTs were associated with more severe tubulitis and interstitial fibrosis/tubular atrophy at 12 months and predicted poorer graft function independently from the degree of interstitial inflammation. Pretransplantation rituximab treatment dramatically attenuated the development of stage II TLTs.

CONCLUSIONS

TLTs are commonly found in clinically stable transplanted kidneys. Advanced stage II TLTs are associated with progressive graft dysfunction, independent of interstitial inflammation.

Presence of Donor Lymph Nodes Within Vascularized Composite Allotransplantation Ameliorates VEGF-C-mediated Lymphangiogenesis and Delays the Onset of Acute Rejection

BACKGROUND

The lymphatic system plays an active role in modulating inflammation in autoimmune diseases and organ rejection. In this work, we hypothesized that the transfer of donor lymph node (LN) might be used to promote lymphangiogenesis and influence rejection in vascularized composite allotransplantation (VCA).

METHODS

Hindlimb transplantations were performed in which (1) recipient rats received VCA containing donor LN (D:LN+), (2) recipient rats received VCA depleted of all donor LN (D:LN-), and (3) D:LN+ transplantations were followed by lymphangiogenesis inhibition using a vascular endothelial growth factor receptor-3 (VEGFR3) blocker.

RESULTS

Our data show that graft rejection started significantly later in D:LN+ transplanted rats as compared to the D:LN- group. Moreover, we observed a higher level of VEGF-C and a quicker and more efficient lymphangiogenesis in the D:LN+ group as compared to the D:LN- group. The presence of donor LN within the graft was associated with reduced immunoactivation in the draining LN and increased frequency of circulating and skin-resident donor T regulatory cells. Blocking of the VEGF-C pathway using a VEGFR3 blocker disrupts the lymphangiogenesis process, accelerates rejection onset, and interferes with donor T-cell migration.

CONCLUSIONS

This study demonstrates that VCA LNs play a pivotal role in the regulation of graft rejection and underlines the potential of specifically targeting the LN component of a VCA to control graft rejection.

Heterogeneity in tertiary lymphoid structure B-cells correlates with patient survival in metastatic melanoma

BACKGROUND

Tertiary lymphoid structures (TLSs) are immune aggregates in peripheral tissues that may support adaptive immune responses. Their presence has been associated with clinical response to checkpoint blockade therapy (CBT), but it is unknown whether TLS have prognostic significance independent of CBT in melanoma. We hypothesized that TLS in melanoma metastases would be associated with increased intratumoral lymphocyte infiltration, but that the intra-TLS immunological milieu would be distinct from the intratumoral immunological milieu. We also hypothesized that the presence of TLS would be associated with improved survival, and that TLS maturation or intra-TLS lymphocyte activity would also correlate with survival.

METHODS

Cutaneous melanoma metastases (CMM) from 64 patients were evaluated by multiplex immunofluorescence for the presence and maturation status of TLS. Intra-TLS lymphocyte density, proliferation and B-cell Ig somatic hypermutation (AID+) were analyzed, as were markers of T-cell exhaustion and Th1/Tc1 differentiation. Associations between TLS maturation and intra-TLS immunologic activity were assessed, as well as associations with intratumoral immune cell infiltration. Independent associations with overall survival (OS) were assessed using log-rank tests and Cox proportional hazards models.

RESULTS

TLS were identified in 30 (47%) of 64 CMM (TLS+) and were associated with increased intratumoral lymphocyte infiltration. However, proliferation of intra-TLS lymphocytes did not correlate with intratumoral lymphocyte proliferation. Most were early TLS; however, subsets of primary or secondary follicle-like TLS were also present. TLS+ lesions were associated with lower risk of tumor recurrence after metastasectomy and with improved OS in multivariate analyses (HR 0.51, p=0.04). OS was longer for TLS with low fractions of CD21+ B-cells (HR 0.29, p=0.02) and shorter for those with low AID+ fraction of B-cells (HR 2.74, p=0.03).

CONCLUSIONS

The presence of TLS in CMMs is associated with improved OS in patients treated with surgery before CBT, but TLS vary widely in maturation state, in proportions of proliferating T and B cells, and in markers of B cell function, including AID and CD21. Importantly, these features have additional prognostic significance, which suggest that some TLS may have regulatory function, while others functioning to support antigen-driven immune responses, depending on the cellular composition and activation status.

Tertiary Lymphoid Structures: Diversity in Their Development, Composition, and Role

Lymph node stromal cells coordinate the adaptive immune response in secondary lymphoid organs, providing both a structural matrix and soluble factors that regulate survival and migration of immune cells, ultimately promoting Ag encounter. In several inflamed tissues, resident fibroblasts can acquire lymphoid-stroma properties and drive the formation of ectopic aggregates of immune cells, named tertiary lymphoid structures (TLSs). Mature TLSs are functional sites for the development of adaptive responses and, consequently, when present, can have an impact in both autoimmunity and cancer conditions. In this review, we go over recent findings concerning both lymph node stromal cells and TLSs function and formation and further describe what is currently known about their role in disease, particularly their potential in tolerance.

Lymphatic drainage from bronchus-associated lymphoid tissue in tolerant lung allografts promotes peripheral tolerance

Tertiary lymphoid organs are aggregates of immune and stromal cells including high endothelial venules and lymphatic vessels that resemble secondary lymphoid organs and can be induced at nonlymphoid sites during inflammation. The function of lymphatic vessels within tertiary lymphoid organs remains poorly understood. During lung transplant tolerance, Foxp3+ cells accumulate in tertiary lymphoid organs that are induced within the pulmonary grafts and are critical for the local downregulation of alloimmune responses. Here, we showed that tolerant lung allografts could induce and maintain tolerance of heterotopic donor-matched hearts through pathways that were dependent on the continued presence of the transplanted lung. Using lung retransplantation, we showed that Foxp3+ cells egressed from tolerant lung allografts via lymphatics and were recruited into donor-matched heart allografts. Indeed, survival of the heart allografts was dependent on lymphatic drainage from the tolerant lung allograft to the periphery. Thus, our work indicates that cellular trafficking from tertiary lymphoid organs regulates immune responses in the periphery. We propose that these findings have important implications for a variety of disease processes that are associated with the induction of tertiary lymphoid organs.

The Implications of B-lineage Cells in Kidney Allografts

The majority of cells comprising the inflammatory infiltrates in kidney allografts undergoing acute and/or chronic rejection are typically T cells and monocyte/macrophages with B cells, plasma cells, and eosinophils accounting for <5%. In a significant minority of biopsies, B lineage cells (B cells and/or plasma cells) may be found more abundantly. Although plasma cell infiltrates tend to be more diffuse, B cells tend to aggregate into nodules that may mature into tertiary lymphoid organs. Given the ability to target B cells with anti-CD20 monoclonal antibodies and plasma cells with proteasome inhibitors and anti-CD38 monoclonal antibodies, it is increasingly important to determine the significance of such infiltrates. Both cell types are potential effectors of rejection, but both also have a tolerizing potential. B cell infiltrates have been associated with steroid resistance and reduced graft survival in some studies but not in others, and their presence should not prompt automatic depletional therapy. Plasma cell-rich infiltrates tend to occur later, may be associated with cell-mediated and/or antibody-mediated rejection, and portend an adverse outcome. Viral infection and malignancy must be ruled out. Randomized controlled trials are needed to determine the appropriateness of specific therapy when B cells and/or plasma cells are found. No strong therapeutic recommendations can be made at this time.

Activated mesangial cells acquire the function of antigen presentation

Mesangial cells (MCs), as resident cells of the kidneys, play an important role in maintaining glomerular function. MCs are located between the capillary loops of the glomeruli and mainly support the capillary plexus, constrict blood vessels, extracellular matrix components, produce cytokines, and perform phagocytosis and clearance of macromolecular substances. When the glomerular environment changes, MCs are often affected, which can lead to functional transformation. The immune response is involved in the occurrence and development of various kidney diseases, in these diseases, antigen-presenting cells (APCs) play an important role. APCs can present antigens to T lymphocytes, causing them to become activated and proliferate. Studies have shown that MCs have phagocytic function and express APC markers on the cell surface. Additionally, MCs are stimulated by or produce various inflammatory factors to participate in the renal inflammatory response. Therefore, MCs have potential antigen presentation function and participate in the pathological changes of various kidney diseases as APCs upon activation. In this paper, by reviewing MC phagocytic function, activated MC expression of APC surface markers, and MC participation in the inflammatory response and local renal immune response, we confirm that activated MCs can act as APCs in renal disease.

Donor-host Lymphatic Anastomosis After Murine Lung Transplantation

BACKGROUND

Establishing lung lymphatic drainage is thought to be important for successful lung transplantation. To date, there has been a complete absence of knowledge of how lymphatic connections are reestablished after lung transplant, despite evidence suggesting that this does indeed occur. The present study aimed to elucidate whether and how lymphatic anastomosis occurs after lung transplant.

METHODS

An orthotopic murine model of lung transplant using lymphatic reporter mice and whole mount immunohistochemistry was used to evaluate the lymphatic vasculature and donor-host connections after lung transplantation.

RESULTS

Immunohistochemistry of transplanted lungs demonstrated robust lymphatic vessels, and functional assays demonstrated lymphatic drainage in the transplanted lung that was comparable with that in native lungs. Lymphatic vessels in the donor lung exhibited active sprouting toward the host at the anastomosis within the first 3 days after lung transplantation, with more numerous and complex lymphatic sprouting developing thereafter. Donor lymphatic vessels were numerous at the site of anastomosis by day 14 after lung transplantation and formed physical connections with host lymphatic vessels, demonstrating a mechanism by which lymphatic drainage is reestablished in the transplanted lung.

CONCLUSIONS

Lymphatic drainage after lung transplantation is established by active sprouting of donor lymphatic vessels towards the host and the formation of donor-host lymphatic connections at the level of the transplant anastomosis.

Developmental stages of tertiary lymphoid tissue reflect local injury and inflammation in mouse and human kidneys

Tertiary lymphoid tissues (TLTs) are inducible ectopic lymphoid tissues in chronic inflammatory states and function as sites of priming local immune responses. We previously demonstrated that aged but not young mice exhibited multiple TLTs after acute kidney injury and that TLTs were also detected in human aged and diseased kidneys. However, the forms of progression and the implication for kidney injury remain unclear. To clarify this we analyzed surgically resected kidneys from aged patients with or without chronic kidney disease as well as kidneys resected for pyelonephritis, and classified TLTs into three distinct developmental stages based on the presence of follicular dendritic cells and germinal centers. In injury-induced murine TLT models, the stages advanced with the extent of kidney injury, and decreased with dexamethasone accompanied with improvement of renal function, fibrosis and inflammation. Kidneys from aged patients with chronic kidney disease consistently exhibited more frequent and advanced stages of TLTs than those without chronic kidney disease. Kidneys of patients with pyelonephritis exhibited more frequent TLTs with more advanced stages than aged kidneys. Additionally, TLTs in both cohorts shared similar locations and components, suggesting that TLT formation may not be a disease-specific phenomenon but rather a common pathological process. Thus, our findings provide the insights into biological features of TLT in the kidney and implicate TLT stage as a potential marker reflecting local injury and inflammation.

IL-22 is required for the induction of bronchus-associated lymphoid tissue in tolerant lung allografts

Long-term survival after lung transplantation remains profoundly limited by graft rejection. Recent work has shown that bronchus-associated lymphoid tissue (BALT), characterized by the development of peripheral nodal addressin (PNAd)-expressing high endothelial venules and enriched in B and Foxp3+ T cells, is important for the maintenance of allograft tolerance. Mechanisms underlying BALT induction in tolerant pulmonary allografts, however, remain poorly understood. Here, we show that the development of PNAd-expressing high endothelial venules within intragraft lymphoid follicles and the recruitment of B cells, but not Foxp3+ cells depends on IL-22. We identify graft-infiltrating gamma-delta (γδ) T cells and Type 3 innate lymphoid cells (ILC3s) as important producers of IL-22. Reconstitution of IL-22 at late time points through retransplantation into wildtype hosts mediates B cell recruitment into lymphoid follicles within the allograft, resulting in a significant increase in their size, but does not induce PNAd expression. Our work has identified cellular and molecular requirements for the induction of BALT in pulmonary allografts during tolerance induction and may provide a platform for the development of new therapies for lung transplant patients.

Extracellular vesicles derived from injured vascular tissue promote the formation of tertiary lymphoid structures in vascular allografts

Tertiary lymphoid structures (TLS) accumulate at sites of chronic injury where they function as an ectopic germinal center, fostering local autoimmune responses. Vascular injury leads to the release of endothelial-derived apoptotic exosome-like vesicles (ApoExo) that contribute to rejection in transplanted organs. The purpose of the study was to evaluate the impact of ApoExo on TLS formation in a model of vascular allograft rejection. Mice transplanted with an allogeneic aortic transplant were injected with ApoExo. The formation of TLS was significantly increased by ApoExo injection along with vascular remodeling and increased levels of antinuclear antibodies and anti-perlecan/LG3 autoantibodies. ApoExo also enhanced allograft infiltration by γδT17 cells. Recipients deficient in γδT cells showed reduced TLS formation and lower autoantibodies levels following ApoExo injection. ApoExo are characterized by proteasome activity, which can be blocked by bortezomib. Bortezomib treated ApoExo reduced the recruitment of γδT17 cells to the allograft, lowered TLS formation, and reduced autoantibody production. This study identifies vascular injury-derived extracellular vesicles (ApoExo), as initiators of TLS formation and demonstrates the pivotal role of γδT17 in coordinating TLS formation and autoantibody production. Finally, our results suggest proteasome inhibition with bortezomib as a potential option for controlling TLS formation in rejected allografts.

Characterizing the B-Cell and Humoral Response in Tertiary Lymphoid Organs in Kidney Allografts

OBJECTIVES

Tertiary lymphoid organs are formed at sites of chronic inflammation and are thought to contribute to the immune response. Here, we aimed to characterize the structure and function of tertiary lymphoid organs in a model of murine kidney allotransplant to understand their role in alloimmunity.

MATERIALS AND METHODS

We transplanted 4 C57BL/6 mouse kidneys (isograft group) and 17 DBA/2 mouse kidneys into C57BL/6 mouse recipients. Three DBA/2-to-C57BL/6 transplant mice that rejected their grafts acutely (before 10 days posttransplant) were excluded from the study. The 14 surviving DAB2 grafts were retrieved at day 45 posttransplant and evaluated histologically. The presence of antibody-secreting cells and circulating levels of donor-specific antibodies were also evaluated.

RESULTS

We found that tertiary lymphoid organs can be associated with a beneficial response in a kidney allotransplant model. Characterization of B-cell subsets within tertiary lymphoid organs in mouse kidney allografts revealed naive, plasma, and memory B cells, which were mostly grouped within or in close proximity of tertiary lymphoid organs. Staining for intracellular immunoglobulin G showed that many of the B cells within tertiary lymphoid organs were capable of producing antibodies. Although allospecific antibodies were found in the serum of recipient mice and were deposited in the transplanted kidneys, graft function was not affected in this model.

CONCLUSIONS

B cells within tertiary lymphoid organs are functional and contribute to the humoral arm of the alloresponse. However, tertiary lymphoid organs are not necessarily associated with graft rejection, suggesting that protective mechanisms are at play.

Bronchus-associated lymphoid tissue-resident Foxp3+ T lymphocytes prevent antibody-mediated lung rejection

Antibody-mediated rejection (AMR) is a principal cause of acute and chronic failure of lung allografts. However, mechanisms mediating this oftentimes fatal complication are poorly understood. Here, we show that Foxp3+ T cells formed aggregates in rejection-free human lung grafts and accumulated within induced bronchus-associated lymphoid tissue (BALT) of tolerant mouse lungs. Using a retransplantation model, we show that selective depletion of graft-resident Foxp3+ T lymphocytes resulted in the generation of donor-specific antibodies (DSA) and AMR, which was associated with complement deposition and destruction of airway epithelium. AMR was dependent on graft infiltration by B and T cells. Depletion of graft-resident Foxp3+ T lymphocytes resulted in prolonged interactions between B and CD4+ T cells within transplanted lungs, which was dependent on CXCR5-CXCL13. Blockade of CXCL13 as well as inhibition of the CD40 ligand and the ICOS ligand suppressed DSA production and prevented AMR. Thus, we have shown that regulatory Foxp3+ T cells residing within BALT of tolerant pulmonary allografts function to suppress B cell activation, a finding that challenges the prevailing view that regulation of humoral responses occurs peripherally. As pulmonary AMR is largely refractory to current immunosuppression, our findings provide a platform for developing therapies that target local immune responses.

Spectrum of chronic lung allograft pathology in a mouse minor-mismatched orthotopic lung transplant model

Chronic lung allograft dysfunction (CLAD) is a fatal condition that limits survival after lung transplantation (LTx). The pathological hallmark of CLAD is obliterative bronchiolitis (OB). A subset of patients present with a more aggressive CLAD phenotype, called restrictive allograft syndrome (RAS), characterized by lung parenchymal fibrosis (PF). The mouse orthotopic single LTx model has proven relevant to the mechanistic study of allograft injury. The minor-alloantigen-mismatched strain combination using C57BL/10(B10) donors and C57BL/6(B6) recipients reportedly leads to OB. Recognizing that OB severity is a spectrum that may coexist with other pathologies, including PF, we aimed to characterize and quantify pathologic features of CLAD in this model. Left LTx was performed in the following combinations: B10→B6, B6→B10, B6→B6. Four weeks posttransplant, blinded pathologic semi-quantitative assessment showed that OB was present in 66% of B10→B6 and 30% of B6→B10 grafts. Most mice with OB also had PF with a pattern of pleuroparenchymal fibroelastosis, reminiscent of human RAS-related pathology. Grading of pathologic changes demonstrated variable severity of airway fibrosis, PF, acute rejection, vascular fibrosis, and epithelial changes, similar to those seen in human CLAD. These assessments can make the murine LTx model a more useful tool for further mechanistic studies of CLAD pathogenesis.

Immune Cell Infiltration and Tertiary Lymphoid Structures as Determinants of Antitumor Immunity

Limited representation of intratumoral immune cells is a major barrier to tumor control. However, simply enhancing immune responses in tumor-draining lymph nodes or through adoptive transfer may not overcome the limited ability of tumor vasculature to support effector infiltration. An alternative is to promote a sustained immune response intratumorally. This idea has gained traction with the observation that many tumors are associated with tertiary lymphoid structures (TLS), which organizationally resemble lymph nodes. These peri- and intratumoral structures are usually, but not always, associated with positive prognoses in patients. Preclinical and clinical data support a role for TLS in modulating immunity in the tumor microenvironment. However, there appear to be varied functions of TLS, potentially based on their structure or location in relation to the tumor or the origin or location of the tumor itself. Understanding more about TLS development, composition, and function may offer new therapeutic opportunities to modulate antitumor immunity.

B Cell Immunity in Solid Organ Transplantation

The contribution of B cells to alloimmune responses is gradually being understood in more detail. We now know that B cells can perpetuate alloimmune responses in multiple ways: (i) differentiation into antibody-producing plasma cells; (ii) sustaining long-term humoral immune memory; (iii) serving as antigen-presenting cells; (iv) organizing the formation of tertiary lymphoid organs; and (v) secreting pro- as well as anti-inflammatory cytokines. The cross-talk between B cells and T cells in the course of immune responses forms the basis of these diverse functions. In the setting of organ transplantation, focus has gradually shifted from T cells to B cells, with an increased notion that B cells are more than mere precursors of antibody-producing plasma cells. In this review, we discuss the various roles of B cells in the generation of alloimmune responses beyond antibody production, as well as possibilities to specifically interfere with B cell activation.

Potential of Cells and Cytokines/Chemokines to Regulate Tertiary Lymphoid Structures in Human Diseases

Tertiary lymphoid structures (TLS) are ectopic lymphoid tissues involved in chronic inflammation, autoimmune diseases, transplant rejection and cancer. They exhibit almost all the characteristics of secondary lymphoid organs (SLO), which are associated with adaptive immune responses; as such, they contain organized B-cell follicles with germinal centers, distinct areas containing T cells and dendritic cells, high endothelial venules, and lymphatics. In this review, we briefly describe the formation of SLO, and describe the cellular subsets and molecular cues involved in the formation and maintenance of TLS. Finally, we discuss the associations of TLS with human diseases, especially autoimmune diseases, and the potential for therapeutic targeting.