Analysis of Cross-sectional and Longitudinal HLA and Anti-viral Responses After COVID Infection in Renal Allograft Recipients: Differences and Correlates

BACKGROUND

Characterization of anti-HLA versus anti-severe acute respiratory syndrome coronavirus 2 (anti-SARS-CoV-2) immune globulin isotypes in organ transplant recipients after coronavirus disease 2019 (COVID-19) infection has not been reported. We aimed to determine changes in anti-HLA antibodies in renal transplant patients with COVID-19 and compare the immunoglobulin and epitope-binding pattern versus anti-SARS-CoV-2 antibodies.

METHODS

This is a cross-sectional study of 46 kidney transplant recipients including 21 with longitudinal sampling. Using a semi-quantitative multiplex assay, we determined immunoglobulin (Ig) M, IgA, IgG, and IgG1-2-3-4 antibodies against Class I and Class II HLA, and 5 SARS-CoV-2 epitopes including the nucleocapsid protein and multiple regions of the spike protein.

RESULTS

Fourteen of 46 (30%) patients had donor-specific anti-HLA antibodies (donor-specific antibody [DSA]), 12 (26%) had non-DSA anti-HLA antibodies and 45 (98%) had anti-SARS-CoV-2 antibodies. Most DSAs targeted HLA-DQ (71%), with a dominant IgG isotype and IgG1 subtype prevalence (93%), and/or IgG3 (64%), followed by IgG2 (36%). Comparatively, there was a higher prevalence of IgA (85% versus 14%, P = 0.0001) and IgM (87%, versus 36%, P = 0.001) in the anti-SARS-CoV-2 antibody profile, when compared to DSAs, respectively. Anti-SARS-CoV-2 antibody profile was characterized by increased prevalence of IgM and IgA, when compared to DSAs. The median calculated panel reactive antibody before COVID-19 diagnosis (24%) tended to decrease after COVID-19 diagnosis (10%) but it was not statistically significant ( P = 0.1).

CONCLUSIONS

Anti-HLA antibody strength and calculated panel reactive antibody in kidney transplant recipients after COVID-19 do not significantly increase after infection. Although the IgG isotype was the dominant form in both HLA and SARS-CoV-2 antigens, the alloimmune response had a low IgA pattern, whereas anti-SARS-CoV-2 antibodies were high IgA/IgM.

Expression of immune inhibitory receptor ILT3 in acute myeloid leukemia with monocytic differentiation

BACKGROUND

The diagnosis of AML with monocytic differentiation is limited by the lack of highly sensitive and specific monocytic markers. Immunoglobulin-like transcript 3 (ILT3) is an immune inhibitory receptor expressed by myelomonocytic cells and at high levels by tolerogenic dendritic cells.

METHODS

Using flow cytometry, we analyzed the expression of ILT3 in 37 patients with AML and 20 patients with no detectable disease.

RESULTS

We showed that ILT3 was expressed in all cases of AML displaying monocytic differentiation (FAB M4/M5; N = 18), but not in AML M1/M2 and M3 (N = 19; P < 0.0001). Co-expression of ILT3 and immature cell markers, such as CD34 and CD117, was observed in monoblastic leukemia. ILT3 expression was preserved after treatment in M4/M5 patients with refractory or relapsed disease. ILT3 expression was associated with the presence of cytogenetic abnormalities linked to an intermediate prognosis (P = 0.001). Rare CD45dimCD34+CD117+ILT3+ cells were identified in noninvolved bone marrow, suggesting that ILT3 expression is acquired at an early stage by normal myelomonocytic precursors.

CONCLUSIONS

ILT3 is a highly sensitive and specific marker which distinguishes AML with monocytic differentiation from other types of AML. Testing of ILT3 expression should be incorporated into the initial diagnostic work-up and monitoring of patients with AML.

Membrane and soluble ILT3 are critical to the generation of T suppressor cells and induction of immunological tolerance

The tolerogenic phenotype of human dendritic cells is characterized by high cell surface expression of the inhibitory receptor ILT3. ILT3 signals both intracellularly inhibiting tyrosine phosphorylation, NF-kappaB and MAPK p38 activity, transcription of certain co-stimulatory molecules, secretion of cytokines and chemokines, and extracellularly into the T cells with which the dendritic cells interact. Both ILT3(high) tolerogenic dendritic cells and soluble ILT3 induce CD4 Th anergy and differentiation of antigen specific CD8 T suppressor cells. Recombinant ILT3-Fc protein has important immunotherapeutic potential acting directly on activated T cells and promoting the induction of immunological tolerance.

Increased adenosine triphosphate production by peripheral blood CD4+ cells in patients with hematologic malignancies treated with stem cell mobilization agents

Hematopoietic stem cell (HSC) transplantation is an important therapeutic option for patients with hematologic malignancies. To explore the immunomodulatory effects of HSC mobilization agents, we studied the function and phenotype of CD4(+) T cells from 16 adult patients with hematologic malignancies undergoing HSC mobilization treatment for autologous transplantation. Immune cell function was determined using the Immuknow (Cylex) assay by measuring the amount of adenosine triphosphate (ATP) produced by CD4(+) cells from whole blood. ATP activity measured in G-CSF-treated patients was significantly higher than that measured in healthy individuals or "nonmobilized" patients. In patients treated with G-CSF, CD4(+) T cells were predominantly CD25(low)FOXP3(low), consistent with an activated phenotype. However, T-cell depletion did not abrogate ATP production in blood samples from G-CSF-treated patients, indicating that CD4(+) myeloid cells contributed to the increased ATP levels observed in these patients. There was a significant correlation between ATP activity and patient survival, suggesting that efficient activation of CD4(+) cells during mobilization treatment predicts a low risk of disease relapse. Monitoring immune cell reactivity using the Immuknow assay may assist in the clinical management of patients with hematologic malignancies and optimization of HSC mobilization protocols.

Hematogones are markedly reduced in pediatric acquired aplastic anemia: multiparametric flow cytometric analysis

Acquired aplastic anemia (AA) and myelodysplastic syndromes (MDS) are bone marrow (BM) failure syndromes with overlapping clinical features, and at least a subset appears to share common pathophysiologic mechanisms. Recent studies of MDS have shown down-regulation of genes involved in B-cell development and decreased B-cell precursors (hematogones). We explored the possibility that AA, similar to MDS, might also be associated with defects in development of lymphoid cells, especially B-cells, by using flow cytometry to assess the presence of hematogones and mature lymphocytes in BM samples from 25 children with AA and 41 age-matched controls. We observed that the percentage of total and early (stage I) hematogones were significantly decreased in AA compared to controls, and they returned to normal numbers after hematopoietic stem-cell transplant. This demonstrates early B-cell lineage involvement in AA, similar to recent findings in MDS. Our findings suggest dysfunction of an early multilineage progenitor in the pathogenesis of AA.

Suppression of xenogeneic graft-versus-host disease by treatment with immunoglobulin-like transcript 3-Fc

Allogeneic hematopoietic cell transplantation represents an important therapy for certain malignant and nonmalignant diseases. However, graft-versus-host disease (GVHD) is a major cause of mortality and morbidity. The search for agents that can efficiently suppress GVHD has been going on for more than half a century. GVHD is particularly strong in xenogeneic donor-recipient combinations, given the unlimited number of potentially immunogenic antigens donor lymphocytes encounter in the host. Using a hu-nonobese diabetic/severe combined immunodeficiency (hu-NOD/SCID) gamma-null model of xenogeneic GVHD, we have demonstrated that treatment with recombinant immunoglobulin-like transcript 3-Fc protein induces the differentiation of CD8(+) T suppressor cells and blocks the cellular and humoral arm of the GVH reaction.

Use of the haematopoietic progenitor cell parameter in optimizing timing of peripheral blood stem cell harvest

BACKGROUND AND OBJECTIVES

Timing of peripheral blood stem cell (PBSC) harvest is typically based on quantification of peripheral blood (PB) CD34+ cells. CD34 enumeration is expensive, requires expertise and takes a minimum of 1-2 h to perform. The Sysmex XE2100 is an automated haematology analyser that can rapidly and inexpensively identify haematopoietic progenitor cell (HPC) populations in PB. The aim of this study was to examine if HPC can be used to optimize timing of PBSC harvest.

MATERIALS AND METHODS

White blood cell (WBC), HPC and CD34 counts were determined in a total of 60 mobilized donors. Data were analysed to examine the utility of WBC and HPC counts in predicting preharvest CD34+ counts.

RESULTS

In adults presenting for autologous collection, a PB HPC threshold of > 30/microl predicts a preharvest CD34+ count of > 20/microl with sensitivity of 86% and positive predictive value (PPV) of 100%. Among paediatric patients with a diagnosis of neuroblastoma, an HPC threshold of > 16/microl yielded sensitivity and PPV of 100%, while in children with other diagnoses, an HPC cut-off of > 44/microl yielded sensitivity and PPV of 67% and 100%, respectively. Eighty per cent of adequately mobilized allogeneic donors were identified using an HPC threshold > 15/microl, with a PPV of 100%. PB WBC can also aid in predicting CD34 counts in most patient groups, albeit with lower sensitivity than HPC.

CONCLUSION

By virtue of being a sensitive and accurate predictor of preharvest CD34+ counts, our data support the use of the HPC parameter in optimizing the timing of PBSC harvest.

Diffuse large B-cell lymphoma with TEL/ETV6 translocation

Cytogenetic abnormalities of chromosome 12p involving the TEL/ETV6 gene are observed in a variety of hematopoietic neoplasms including acute leukemias, myelodysplastic syndromes, and myeloproliferative disorders. Karyotypic aberrations, including rearrangements, deletions, and amplifications of chromosome 12p, have been documented in B-cell non-Hodgkin lymphoma; however, rearrangements targeting TEL have rarely been reported. Here we describe a diffuse large B-cell lymphoma that had a complex karyotype including t(9;12)(q22;p13), which was confirmed by fluorescence in situ hybridization to represent rearrangement of TEL. Additional cytogenetic abnormalities included t(3;14)(q27;q32) involving the variant, alternative breakpoint region of the BCL6 gene and del(6)(q13q23), resulting in the loss of 1 allele of BLIMP1. This case reiterates the importance of correlating morphologic and phenotypic findings with the results of cytogenetic analysis to avoid errors in diagnosing hematologic neoplasms and highlights the rare association of B-cell non-Hodgkin lymphoma with aberrations of TEL.

Expression of inhibitory receptor ILT3 on neoplastic B cells is associated with lymphoid tissue involvement in chronic lymphocytic leukemia

T cell responses against leukemia-associated antigens have been reported in chronic lymphocytic leukemia (CLL). However, the relentless accumulation of CLL B cells in some patients indicates that anti-tumor immune responses are inefficient. Inhibitory receptors from the Ig-like transcript (ILT) family, such as ILT3 and ILT4, are crucial to the tolerogenic activity of antigen presenting cells. In this study, we examined the expression of ILT3 on CD5+ B cells obtained from 47 patients with CLL. Using flow cytometry and RT-PCR, we found that B CLL cells from 23 of 47 patients expressed ILT3 protein and mature ILT3 mRNA. ILT3 protein and mRNA were not found in normal B cells obtained from donors without CLL. Expression of ILT4 in normal and B CLL cells showed a pattern similar to ILT3. The frequency of ILT3 positive CLL B cells was higher in patients with lymphoid tissue involvement, suggesting that ILT3 may have prognostic value in CLL. Our findings indicate that expression of ILT3 and ILT4 on CLL B cells represents a phenotypic abnormality that may play a role in tolerization of tumor-specific T cells.

G6b-B cell surface inhibitory receptor expression is highly restricted to CD4+ T-cells and induced by interleukin-4–activated STAT6 pathway

The G6b-B gene encodes a novel cell surface receptor of the immunoglobulin superfamily that activates inhibitory signaling pathways by triggering SHP-1/SHP-2 via immunoreceptor tyrosine-based inhibitory motifs (ITIM) in its cytoplasmic domain. We previously identified decreased G6b-B expression in peripheral blood mononuclear cells (PBMC) during acute cellular cardiac allograft rejection. We studied the expression of G6b-B in different human mononuclear cell populations and its regulation. Real-time polymerase chain reaction (PCR) revealed that G6b-B mRNA is higher in CD4+ T cells or monocytes, but is not different between CD25+ CD4+ T cells and CD25- CD4+ T cells. G6b-B mRNA was increased in CD4+ T cells in presence of interleukin-4 in dose- and time-dependent manners. To understand the regulatory mechanism, we analyzed a 1.9-kb 5'-flanking region of the G6b-B translation start site and found a putative cis-acting element for Signal Transducer and Activator of Transcription (STAT)-6. Luciferase-reporter-gene-assay and electrophoretic mobility shift assays identified the STAT6 site as necessary for the induction of G6b-B by IL-4. Our study demonstrates that G6b-B expression is highly restricted to peripheral CD4+ T cells and up-regulated by the IL-4-induced STAT6 pathway, strongly suggesting that G6b-B is involved in regulation of the immune response by CD4+ T cell-mediated and IL-4 induced regulatory mechanisms.

Soluble Ig-like transcript 3 inhibits tumor allograft rejection in humanized SCID mice and T cell responses in cancer patients

Attempts to enhance patients' immune responses to malignancies have been largely unsuccessful. We now describe an immune-escape mechanism mediated by the inhibitory receptor Ig-like transcript 3 (ILT3) that may be responsible for such failures. Using a humanized SCID mouse model, we demonstrate that soluble and membrane ILT3 induce CD8(+) T suppressor cells and prevent rejection of allogeneic tumor transplants. Furthermore, we found that patients with melanoma, and carcinomas of the colon, rectum, and pancreas produce the soluble ILT3 protein, which induces the differentiation of CD8(+) T suppressor cells and impairs T cell responses in MLC. These responses are restored by anti-ILT3 mAb or by depletion of soluble ILT3 from the serum. Immunohistochemical staining of biopsies from the tumors and metastatic lymph nodes suggests that CD68(+) tumor-associated macrophages represent the major source of soluble ILT3. Alternative splicing, resulting in the loss of the ILT3 transmembrane domain, may contribute to the release of ILT3 in the circulation. These data suggest that ILT3 depletion or blockade is crucial to the success of immunotherapy in cancer. In contrast, the inhibitory activity of soluble ILT3 on T cell alloreactivity in vitro and in vivo suggests the potential usefulness of rILT3 for immunosuppressive treatment of allograft recipients or patients with autoimmune diseases.

Alloantibodies and the outcome of cadaver kidney allografts

The role of humoral immunity in causing antibody-mediated rejection (AMR) of organ allografts has been extensively documented. For this reason, negative complement-dependent cytotoxicity (CDC) cross-matches between recipient sera and donor T and B lymphocytes have become a mandatory requirement for cadaveric kidney transplantation. However, the significance of donor-specific antibodies (DSAs) detectable only by flow cytometry (FC) or solid phase assays (SPA) but not CDC is still controversial. We have performed a retrospective analysis of FC cross-matching results in 80 consecutive cadaver kidney allograft recipients. Antibodies against HLA class I and class II antigens were measured by CDC and SPA in sequential samples of sera obtained prior to transplantation. The preoperative cross-match was performed by CDC using magnetically sorted T and B cells from donor spleen. Sera obtained from each patient before and at the time of transplantation were included in the final cross-match. The sample of serum obtained at the time of transplantation was cross-matched retrospectively by FC and analyzed for anti-HLA antibody specificity on high resolution SPA. The actuarial kidney allograft survival at one year was 98%. Two of these eighty patients lost the graft, one due to AMR, the other for reasons unrelated to DSAs. Donor-specific antibodies were detected by FC in 17 of 80 patients, yet only 6 of 17 had an early episode of AMR. This episode was successfully reversed by desensitization therapy using intravenous immunoglobin (IVIG) and plasmapheresis. Flow cytomery cross-matching showed 95% specificity but only 35% sensitivity for prediction of AMR (p = 0.002). There was a significant correlation between high panel reactive antibodies (PRA) and positive FC cross-matching (p = 0 .0001), as well as high PRA and AMR (p = 0.0004 by CDC and 0.0011 by Luminex). Reversible AMR occurred 12-30 days post-transplantation in 8 patients. Of these 8 patients, 3 had no detectable DSAs in spite of C4d positivity, 4 had C4d deposition in conjunction with anti-HLA antibodies, and 1 patient had DSAs (anti-MICA) yet no C4d deposition. We conclude that early initiation of desensitization protocols can prevent transplant failure and that retrospective FC cross-matches may facilitate the diagnosis of AMR. Extensive analysis of patients' sera using a comprehensive set of tests may contribute to early treatment and better understanding of the mechanism underlying humoral rejection.

CUTLL1, a novel human T-cell lymphoma cell line with t(7;9) rearrangement, aberrant NOTCH1 activation and high sensitivity to gamma-secretase inhibitors

Activating mutations in NOTCH1 are present in over 50% of human T-cell lymphoblastic leukemia (T-ALL) samples and inhibition of NOTCH1 signaling with gamma-secretase inhibitors (GSI) has emerged as a potential therapeutic strategy for the treatment of this disease. Here, we report a new human T-cell lymphoma line CUTLL1, which expresses high levels of activated NOTCH1 and is extremely sensitive to gamma-secretase inhibitors treatment. CUTLL1 cells harbor a t(7;9)(q34;q34) translocation which induces the expression of a TCRB-NOTCH1 fusion transcript encoding a membrane-bound truncated form of the NOTCH1 receptor. GSI treatment of CUTLL1 cells blocked NOTCH1 processing and caused rapid clearance of activated intracellular NOTCH1. Loss of NOTCH1 activity induced a gene expression signature characterized by the downregulation of NOTCH1 target genes such as HES1 and NOTCH3. In contrast with most human T-ALL cell lines with activating mutations in NOTCH1, CUTLL1 cells showed a robust cellular phenotype upon GSI treatment characterized by G1 cell cycle arrest and increased apoptosis. These results show that the CUTLL1 cell line has a strong dependence on NOTCH1 signaling for proliferation and survival and supports that T-ALL patients whose tumors harbor t(7;9) should be included in clinical trials testing the therapeutic efficacy NOTCH1 inhibition with GSIs.

CD117 expression in diffuse large B-cell lymphomas: fact or fiction?

CD117 (KIT) is expressed in a variety of hematopoietic neoplasms but there are a paucity of data regarding its expression in diffuse large B-cell lymphomas (DLBCL). The purpose of the present paper was to describe the authors' experience of two CD117+ DLBCL (one of follicle center-cell origin and one nasal Epstein-Barr virus (EBV)- plasmablastic lymphoma associated with lytic bone lesions), as determined by tissue immunohistochemistry and flow cytometry. The CD117 expression in DLBCL was further evaluated using tissue microarrays and seven additional plasmablastic lymphomas, using two commercially available anti-CD117 antibodies (Ab-1, Oncogene and A4502, DakoCytomation). Membranous +/- cytoplasmic staining was seen with Ab-1 in 24/65 (37%) DLBCL, including 21/56 microarray DLBCL, two index cases, and 1/7 additional plasmablastic lymphomas, with persistent staining in 13% of microarray DLBCL despite preincubation with KIT peptide. However, A4502 had only membranous staining of the index cases and one additional EBV- plasmablastic lymphoma with medullary disease. The present study suggests that (i) CD117 expression can be detected sporadically in DLBCL of follicle center-cell origin and a subset of plasmablastic lymphomas; (ii) staining for CD117 might help in identifying EBV- plasmablastic lymphomas associated with bone marrow involvement; and (iii) CD117 antibodies should be carefully validated prior to use, because non-specific staining, as observed with Ab-1, could lead to false-positive results.

Characterizing CD43 expression in haematogones using multicolor flow cytometric analysis

Haematogones are precursor B cells commonly detected in small numbers in the bone marrow. Morphologically, haematogones can mimic lymphoblasts and are best distinguished using multicolour flow cytometry with antibody combinations. Haematogones show characteristic and reproducible patterns of antigen expression representing the B-cell maturation sequence. CD43 expression, widely seen in haematopoietic elements, has not been well characterized in haematogones. We demonstrate that CD43 is consistently expressed in haematogones in a reproducible pattern similar to that of CD10 when combined with CD20. We propose that in combination with other markers, CD43 can be useful in the identification of haematogones.

Acute and hyperacute humoral rejection in kidney allograft recipients treated with anti-human thymocyte antibodies

Equine and rabbit antihuman thymocyte globulins (ATGs) have been used in renal transplantation for prevention and treatment of acute rejection. We now report that hyperacute and acute antibody-mediated rejection of renal allografts occurred in three newly transplanted patients who received ATG for induction therapy. Antibody studies performed using complement-dependent cytotoxicity, flow cytometry, enzyme-linked immunosorbent assay, and Luminex yielded negative results for antilymphocytic and antiendothelial cell antibodies in the pretransplant sera obtained from these patients. ATG treatment was initiated at the time of transplantation. One of the patients experienced hyperacute rejection and required transplant nephrectomy within 24 h of transplantation. The other two patients developed acute antibody-mediated rejection within 14 days after transplantation. None of the patients developed antihuman leukocyte antigen antibodies when humoral rejection occurred. However, xenoantibodies that strongly bound to human lymphocytes and, importantly, to activated endothelial cells, were identified in the sera obtained at the time of humoral rejection. Hence, our results strongly implicate ATG in the induction of antibody-mediated rejection of kidney allografts. Flow cytometry testing of ATG reactivity to endothelial cells may be useful in identifying and discarding the ATG lots containing xenoantibodies that can bind to activated endothelial cells of the transplant.

Flow cytometric analysis of normal and reactive spleen

Spleen is surgically removed for both non-neoplastic and neoplastic pathologies. A significant proportion of splenectomy specimens require distinguishing between reactive and neoplastic conditions (eg lymphoma). To establish a 'normal' reference range for the spleen lymphocyte subsets, fresh samples of benign, reactive spleens obtained from adult patients (N=12) and samples of normal spleen obtained from cadaveric transplant donors (N=14) were analyzed using three- and four-color flow cytometry. Study of pan-B, -T, and -NK marker expression revealed that the frequency of T cells is higher and that of B cells is lower in reactive (non-neoplastic) compared to normal (cadaveric) spleen. Furthermore, our study established a frame of reference for cell markers commonly used for immunophenotyping of lymphoma, and identified discrete lymphocyte subsets, such as early plasma cells and T cells carrying the phenotype of the NK/T subset. These results will facilitate an accurate interpretation of the flow cytometric analysis of human spleen lymphocytes.

Regulatory CD8+CD28- T cells in heart transplant recipients

Human regulatory CD8+CD28- T cells (Ts) generated in vitro were demonstrated to suppress the activation and proliferation of T helper cells (Th) induced by allogeneic cells. This effect requires cell-to-cell contact, is antigen-specific, and results in Th anergy. To study the population of CD8+CD28- T cells present in vivo, flow cytometry was performed on whole blood specimens obtained from 25 heart transplant recipients and 12 normal controls. A significant expansion of CD8+CD28- T cells was found in transplant recipients as compared with normal individuals (p = 0.005). Expression of CD38, human leukocyte antigen-DR, and perforin positive cells within the CD8+CD28- subset was significantly higher in transplant patients than in normal controls, yet there was no correlation between the expression of these markers and acute rejection. Expression of the CD27 marker, however, was significantly higher within CD8+CD28- T cells from patients without rejection as compared with patients in rejection (p = 0.005), indicating that the memory-like CD8+CD28-CD27+ T-cell subset comprises regulatory cells, which play a protective role for the graft. CD8+CD28- T cells isolated from transplant patients did not display cytotoxic activity against donor cells and showed high expression of the killing inhibitory receptor CD94. This study identifies the phenotypic changes that occur in patients with heart transplants and opens new avenues for the induction of specific immunosuppression in transplantation.

Tolerization of dendritic cells by T(S) cells: the crucial role of inhibitory receptors ILT3 and ILT4

Immunoglobulin-like transcript 3 (ILT3) and ILT4 belong to a family of inhibitory receptors expressed by human monocytes and dendritic cells. We show here that CD8+CD28(-) alloantigen-specific T suppressor (TS) cells induce the up-regulation of ILT3 and ILT4 on monocytes and dendritic cells, rendering these antigen-presenting cells (APCs) tolerogenic. Tolerogenic APCs show reduced expression of costimulatory molecules and induce antigen-specific unresponsiveness in CD4+ T helper cells. Studies of human heart transplant recipients showed that rejection-free patients have circulating TS cells, which induce the up-regulation of ILT3 and ILT4 in donor APCs. These findings demonstrate an important mechanism of immune regulation.

Human xenospecific T suppressor cells inhibit T helper cell proliferation to porcine aortic endothelial cells, and NF-kappaB activity in porcine APC

Human T suppressor cells (Ts), capable of preventing autologous T helper cells (Th) from reacting against xenogeneic pig endothelial cells and pig APC can be generated in vitro. Ts derive from a population of CD3(+)CD8(+)CD28(-) T lymphocytes and specifically recognize the MHC class I antigens of the APC used for in vitro immunization. To study the mechanism that underlies suppression, we investigated whether Ts inhibit the expression of costimulatory molecules in xenogeneic professional and semiprofessional APC. We found that Ts down-regulate Th-induced expression of CD86 in pig APC, and that this effect occurs at the level of transcription, as indicated by nuclear run-on and Northern blot assays. EMSA results revealed that inhibition of CD86 expression is mediated by inactivation of transcription factor NF-kappaB. Furthermore, transfection of pig APC with a vector expressing NF-kappaB p65 partially rescued Th-induced expression of the CD86 molecule. These results strongly support the concept that xenospecific Ts inhibit the APC function of xenogeneic cells by preventing activation of NF-kappaB.

Cloning and functional characterization of the 5'-regulatory region of the human CD86 gene

The induction of CD86 expression by IFN-gamma on the surface of various antigen presenting cells has been previously reported. In order to understand the mechanisms by which the expression of the CD86 gene is regulated by IFN-gamma at the transcriptional level, we have cloned and characterized the 5'-flanking region of the human CD86 gene. To functionally analyze the upstream regulatory region of the CD86 gene, a series of luciferase reporter gene constructs were prepared and used for transfection of cells from the monocytic line U937 and Raji B cell line. Under basal conditions, functional activity of these constructs was detected in Raji cells, which show high constitutive expression of the CD86 molecule, but not in U937 cells, which show low expression of CD86 in non-activated state. Induction of CD86 expression by stimulation of U937 cells with IFN-gamma revealed the presence of two functional GAS (gamma-interferon activation site) elements. Gel mobility shift assays showed that these two GAS elements specifically bind an IFN-gamma-induced transcriptional complex. The DNA-protein complex was supershifted by antibody to Stat1 alpha (signal transducer and activator of transcription), but not by antibodies to Stat 2, Stat 3 and Sp1, indicating that GAS elements interact with Stat1 alpha. Point mutations in the GAS elements prevented the formation of DNA-protein complex and significantly reduced the responsiveness of the reporter gene to IFN-gamma. These findings suggest that two functional GAS elements within the human CD86 promoter play an important role in the induction of CD86 gene by binding to IFN-gamma-induced Stat1 alpha.

Induction of xenoreactive CD4+ T-cell anergy by suppressor CD8+CD28- T cells

BACKGROUND

The underlying mechanism of immune suppression mediated by regulatory T cells is not completely understood. In previous studies we have shown that antigen-specific human T suppressor cells (Ts) can be generated in vitro by multiple rounds of stimulation with allogeneic, xenogeneic, or antigen-pulsed autologous antigen-presenting cells (APC). Human Ts express the CD8+CD28- phenotype and require specific recognition of MHC class I/peptide complexes on the surface of APC to block proliferation of T helper cells (Th). The aim of the present study was to explore the activation requirements of Ts as well as the nature of Th unresponsiveness to xenogeneic (swine) antigens induced by Ts.

METHODS AND RESULTS

We investigated whether specific antigenic stimulation of Ts is required for their ability to inhibit early activation of xenoreactive Th (up-regulation of CD40 ligand). Flow cytometry studies indicated that Ts function required specific recognition of MHC class I on the surface of the stimulating APC. However, neither proliferation nor protein synthesis was required for the ability of Ts to inhibit Th. Ts drastically reduced the capacity of xenoreactive Th cells to produce interleukin (IL)-2 in response to the specific APC, without affecting their surface expression of IL-2 receptor. The suppressor effect that Ts exerted on Th proliferation could not be circumvented by CD40 ligation on the surface of the APC but could be reversed by the addition of exogenous IL-2.

CONCLUSION

These data indicate that Ts induce anergy of xenoreactive human Th cells upon specific recognition of MHC class I antigens. Hence, Ts may prevent the activation of T cell-mediated immune responses against xenogeneic transplants.

Specific suppression of human CD4+ Th cell responses to pig MHC antigens by CD8+CD28- regulatory T cells

Evidence that T cells can down-regulate the immune response by producing or consuming certain cytokines or by lysing APCs or Th cells has been provided in various systems. However, the generation and characterization of suppressor T cell lines have met with limited success. Here we show that xenospecific suppressor T cells can be generated by in vitro stimulation of human T cells with pig APCs. Similar to allospecific suppressors, these xenospecific suppressor T cells carry the CD8+CD28- phenotype and react to MHC class I Ags expressed by the APCs used for priming. TCR spectratyping of T suppressor cells showed oligoclonal usage of TCR-Vbeta families, indicating that xenostimulation of CD8+CD28- T cells results in Ag-driven selection of a limited Vbeta repertoire. Xenospecific T suppressor cells prevent the up-regulation of CD154 molecules on the membrane of Th cells, inhibiting their ability to react against the immunizing MHC class II xenoantigens. The mechanism of this suppression, therefore, appears to be blockade of CD154/CD40 interaction required for efficient costimulation of activated T cells.

Specific Suppression of Human CD4+ Th Cell Responses to Pig MHC Antigens by CD8+CD28− Regulatory T Cells

Evidence that T cells can down-regulate the immune response by producing or consuming certain cytokines or by lysing APCs or Th cells has been provided in various systems. However, the generation and characterization of suppressor T cell lines have met with limited success. Here we show that xenospecific suppressor T cells can be generated by in vitro stimulation of human T cells with pig APCs. Similar to allospecific suppressors, these xenospecific suppressor T cells carry the CD8+CD28− phenotype and react to MHC class I Ags expressed by the APCs used for priming. TCR spectratyping of T suppressor cells showed oligoclonal usage of TCR-Vβ families, indicating that xenostimulation of CD8+CD28− T cells results in Ag-driven selection of a limited Vβ repertoire. Xenospecific T suppressor cells prevent the up-regulation of CD154 molecules on the membrane of Th cells, inhibiting their ability to react against the immunizing MHC class II xenoantigens. The mechanism of this suppression, therefore, appears to be blockade of CD154/CD40 interaction required for efficient costimulation of activated T cells.

Persistent allopeptide reactivity and epitope spreading in chronic rejection of organ allografts

The role of the indirect allorecognition pathway in acute allograft rejection has been documented both in organ recipients and in experimental models. However, it is unknown whether self-restricted recognition of donor alloantigens also contributes to chronic allograft rejection. The aim of this study was to determine the relationship between allopeptide reactivity, epitope spreading, and chronic rejection. Using synthetic peptides corresponding to the hypervariable region of 32 HLA-DR alleles, we have followed the specificity of self-restricted T cell alloresponses to the donor in a population of 34 heart allograft recipients. T cells from sequential samples of blood collected from the patients up to 36 mo after transplantation were studied in limiting dilution analysis for allopeptide reactivity. The incidence of coronary artery vasculopathy (CAV) was significantly higher in patients who displayed persistent alloreactivity late after transplantation than in patients who showed no alloreactivity after the first 6 mo after transplantation. Both intra- and intermolecular spreading of epitopes was observed with an increased frequency in patients developing CAV in less than 2 yr, compared with patients without CAV; this suggests that diversification of the immune response against the graft contributes to chronic rejection. These data provide a strategy for identifying patients at risk of developing CAV and a rationale for therapeutic intervention aimed to prevent the progression of the rejection process.