ALBERT

Description: Patients with hematological malignancies can be successfully treated with allogeneic hematopoietic stem cell transplantation (alloSCT). Beneficial Graft-versus-Leukemia (GvL) reactivity, however, is often accompanied with undesired Graft-versus-Host Disease (GvHD). In HLA-matched alloSCT, donor T-cells can mediate GvL reactivity by recognition of minor histocompatibility antigens (MiHA) on malignant cells, but also GvHD when MiHA are recognized on non-hematopoietic tissues. To decrease the incidence and severity of GvHD, T-cells can be (partially) depleted from the graft and re-administered later as donor lymphocyte infusion (DLI). MiHA are polymorphic peptides that are presented in HLA-molecules and can be recognized as non-self by donor-derived T-cells. Only a minority of MiHA have therapeutic relevance based on hematopoietic-restricted expression and only 25% and 40% of recipients transplanted with sibling and unrelated donors, respectively, are eligible for T-cell therapies in which one of the known hematopoietic-restricted MiHA is targeted. Therefore, to increase the efficacy and limit the toxicity of T-cell therapy, more therapeutic MiHA are needed. Recently, we identified a MiHA encoded by ARHGDIB, a gene that has been described to be highly expressed on hematopoietic cells, and we here studied the therapeutic relevance of LB-ARHGDIB-1R in detail. First, we confirmed hematopoietic-restricted expression of ARHGDIB by microarray gene expression analysis and demonstrated 〉10-fold overexpression in the majority of malignant and healthy hematopoietic versus non-hematopoietic cells. In line with its hematopoietic-restricted gene expression profile, LB-ARHGDIB-1R in the context of HLA-B* 07:02 was specifically recognized on different hematological malignancies, but not on non-hematopoietic fibroblasts and keratinocytes cultured in the absence or presence of IFN-γ, which was added to mimick the pro-inflammatory cytokine milieu of the early post-transplantation period. Next, we investigated the cytolytic capacity of LB-ARHGDIB-1R specific T-cells, and demonstrated specific lysis of patient, but not donor, EBV-B cells and specific lysis of an ALL sample in a 10 hrs 51Cr-release assay. Specific lysis of additional ALL and AML samples could be measured after 48 hrs of co-incubation in a flowcytometry-based cytotoxicity assay. To determine the in vivo immunogenicity of LB-ARHGDIB-1R, 11 MiHA-disparate HLA-B* 07:02 positive patient-donor pairs were screened for specific CD8+ T-cells by dual color tetramer analysis. All patients received partial T-cell depleted alloSCT and sampling was done at different time points after alloSCT (and DLI). In 4 out of 11 patients, LB-ARHGDIB-1R-specific T-cells (〉0.01%) could be detected directly ex vivo and in 4 additional patients after 7 days of in vitro peptide stimulation, indicating that LB-ARHGDIB-1R is highly immunogenic. High frequencies of LB-ARHGDIB-1R specific T-cells were measured ex vivo in a patient whose hematological relapse was successfully treated with DLI, without development of GvHD, further supporting the therapeutic relevance of LB-ARHGDIB-1R. In summary, we confirmed hematopoietic-restricted expression of LB-ARHGDIB-1R and demonstrated T-cell mediated lysis of primary leukemic cells in long-term co-incubation assays. Furthermore, we showed that LB-ARHGDIB-1R is highly immunogenic and that specific T-cells could be detected in a patient who responded to DLI in the absence of GvHD. Altogether, our data support the clinical relevance of LB-ARHGDIB-1R as therapeutic MiHA with the potential to shift the delicate balance between GvL and GvHD in favor of a desired anti-tumor effect. Disclosures: No relevant conflicts of interest to declare.

Description: For the induction or boosting of antigen-specific CD8+ T cell responses, long synthetic peptides have been used in vaccination studies. Superior in vivo CD8+ T cell responses have been reported following vaccination with long peptides compared with minimal peptides, which was attributed to selective uptake and cross-presentation by professional antigen-presenting cells. Furthermore, to generate antigen-specific T cell lines for adoptive immunotherapy or to measure antigen-specific T cell responses, protein-spanning pools of overlapping long synthetic peptides can be used to simultaneously activate CD8+ and CD4+ T cells in peripheral blood mononuclear cells (PBMC) ex vivo. Although exogenous antigen is predominantly presented in MHC class II, it has been suggested that cross-presentation of long peptides in MHC class I can occur. However, the mechanism of cross-presentation of exogenous long peptides in MHC class I is not clear. Various models for cross-presentation have been described following uptake of soluble antigen in endosomes, among which antigen transport over the endosomal membrane followed by the classical proteasome- and TAP-dependent route, and entrance of MHC class I in the recycling endocytic MHC class II pathway where peptidase-trimmed exogenous antigens can exchange with peptides in the MHC class I molecules, resulting in TAP- and proteasome-independent cross-presentation. To improve the design of peptides for the in vivo or ex vivo activation of CD8+ T cells we investigated the mechanism and efficiency of cross-presentation of long peptides. We observed that antigen-presenting cells in peripheral blood, in particular monocytes, loaded with 15-mer peptides, 31-mer peptides or full length protein containing the NLV epitope were able to very efficiently induce IFNg production by cytomegalovirus (CMV) pp65 NLV-specific T cells. Specific T cells were most efficiently activated by N-terminally extended variants of the minimal epitope, while the use of C-terminally extended variants resulted in a 10-fold reduction of activation efficiency. Purification of these antigens by high performance liquid chromatography (HPLC) followed by mass spectrometry demonstrated that activation was not caused by contamination with the minimal epitope sequence. Also CD8+ T cells specific for other CMV and minor histocompatibility antigen (mHag) epitopes were activated by monocytes loaded with 15-mer or 20-mer peptides. Again N-terminally extended variants of minimal epitopes very efficiently induced activation, while the use of C-terminally variants or full length protein resulted in highly variable efficiency of activation, ranging from 10-fold reduction to complete absence of activation. Interestingly, TAP-deficient T2 cells loaded with CMV pp65 NLV antigens also efficiently activated NLV-specific T cells, indicating that the route of presentation was TAP-independent. Addition of lactacystin during loading of monocytes with CMV pp65 NLV 15-mer did not affect activation of specific T cells, suggesting that cross-presentation was proteasome-independent. Addition of primaquine reduced activation of specific T cells by the NLV 15-mer peptide, but not by the minimal NLV 9-mer peptide, suggesting that cross-presentation was dependent on endosomal recycling. To compare cross-presentation with presentation of endogenously synthesized antigen, TAP-competent T1 and TAP-deficient T2 cells were retrovirally transduced with the CMV pp65 gene. CMV pp65-specific T cells were activated by CMV pp65 transduced T1 but not T2 cells, indicating that endogenously synthesized CMV pp65 required processing and presentation by the classical proteasome- and TAP-dependent route. These data suggest that long synthetic peptides can be processed by peptidases in endocytic compartments and presented by recycling MHC class I molecules. Not all immunogenic epitopes that have been selected in vivo for efficient processing and presentation by the classical pathway may be presented efficiently by cross-presentation. As the efficiency of cross-presentation of long synthetic peptides may depend on the sequence of the C-terminal extension, a rational design of peptides is crucial for efficient activation of CD8+ T cells in approaches of vaccination, adoptive transfer and immune monitoring.

Description: Background Cytomegalovirus (CMV) reactivation frequently occurs during early immune reconstitution after hematopoietic stem cell transplantation (HSCT). Although CMV is associated with accelerated immune ageing in healthy individuals, the impact of early CMV reactivation on T-cell immunity long term post HSCT is unknown. In this study, we report the impact of early CMV reactivations on the reconstitution and composition of the T-lymphocyte compartment one to two year after HSCT in a large cohort of pediatric HSCT recipients. Methods We analyzed the lymphocyte compartment one and two year after transplantation in 131 consecutive (2002 - 2011) pediatric HSCT recipients that were eligible for follow up one year post HSCT. Viral infections were routinely monitored by weekly serum viral DNA PCR in the first 100 days. Peripheral blood mononuclear cells were routinely analyzed by multicolor flow cytometry. Six patients with early CMV reactivation and an HLA type for which CMV-tetramers were available were analyzed for the presence and phenotype of CMV-specific CD8+ T-lymphocytes. Results One year post HSCT, patients with early CMV reactivation (n = 46, PCR ≥ 1x ≥ 200 copies / mL) had significantly higher lymphocyte counts compared to patients without CMV reactivation (n = 85, PCR always 〈 200 copies / mL). This could be attributed to a significant increase of CD3+ T-lymphocytes while NK- and B-cell numbers did not differ. Within the T-cell compartment, a three-fold expansion of CD8+ T-cells (median 1323 vs. 424 cells / μL, p 〈 0.0001, Mann-Whitney) and an increase in gamma-delta T-cells (median 104 vs. 47 cells / μL, p = 0.0005) was observed, while absolute numbers of CD4+ T-cells did not differ between the groups. This effect was not observed in relation to Epstein-Barr virus (EBV) or Adenovirus (HAdV) reactivation. In multivariate analysis, CD8+ T-cell numbers one year post HSCT were highly influenced by CMV reactivation (p 〈 0.0001, linear regression) but not affected by pre-transplant CMV serostatus of donor (p = 0.22) or recipient (p = 0.14). In a representative subcohort (n = 53, 2008 - 2010), we more closely analyzed the differentiation stages of T-cells based on expression of CD45RA and CCR7. In both the CD4+ and CD8+T-cell subset, the proportion of Effector Memory (EM) and EMRA T-cells was enlarged in patients with (n = 18) compared to patients without (n = 35) early CMV reactivation. In the CD8+ T-cell compartment, this was caused by a major expansion of CD8+ EM and EMRA T-cells (median 485 vs. 141, p 〈 0.0001 and 509 vs. 114 cells / μL, p 〈 0.0001, Figure A), while the Naive (median 105 vs. 169 cells / μL, p = 0.17) and Central Memory (CM) compartment did not differ. In the CD4+ compartment, a non-significant increase of EM and EMRA cells was accompanied by a non-significant decrease of N and CM cells. CMV-specific CD8+ T-cells (median 4.1, range 0.3 - 25.6% of CD8+ T-cells) were detected within the EM and EMRA compartment. Two year after HSCT, data were available for 76 patients. Both in patients with (lymphocyte subsets: n = 34 / T-cell differentiation: n = 12) and without (n = 42 / n = 19) early CMV reactivation, a further reconstitution of CD4+ and CD8+ T-cells was observed, reflected by an expansion of Naive and CM cells (Figure B). However, the total number of CD8+ T-cells decreased in patients with early CMV reactivation, caused by a contraction of late differentiated CD8+ EM and EMRA cells (median 825 to 618, p = 0.0068 and 555 to 378 cells / μL, p = 0.0342, Wilcoxon). Conclusion Early CMV reactivation leaves a virus-specific and dynamic imprint on the reconstituting immune system 1 and 2 year after HSCT. The marked expansion of CD8 + EM and EMRA T-cells was not seen in patients with early EBV or HAdV reactivation and did not compromise the reconstitution of the Naive and CM compartment available to respond to neo- and recall antigens. Pediatric HSCT recipients differed from solid organ transplantation recipients in which CMV has been correlated to an accelerated and ongoing accumulation of late differentiated T-cells. The dynamic contraction of the CD8+ late differentiated memory T-cell compartment in the second year after HSCT implies that an ongoing process of immune-regulation and further reconstitution is modeling the cellular immune system after discontinuation of immunosuppressive medication. Disclosures: No relevant conflicts of interest to declare.

Description: Patients with the B-cell malignancy hairy cell leukemia (HCL) exhibit a skewed T-cell repertoire with oligoclonal expression or absence of many members of the T-cell receptor (TCR) BV gene families. To evaluate whether interferon-α (IFN-α) therapy would not only restore normal hematopoiesis, but also the abnormal T-cell repertoire, we studied T lymphocytes from a cohort of HCL patients treated by IFN-α in the past, at initiation, and at several intervals up to 6 years of IFN-α treatment. The junctional regions from 22 TCRBV gene families were analyzed after polymerase chain reaction amplification of cDNA (RT-PCR) using family specific primers. In all seven patients improvement of the skewed T-cell repertoire was not seen until 2 years of treatment. It consisted of disappearance of oligoclonal subpopulations and (polyclonal) reappearance of absent TCRBV gene families. The RT-PCR results were correlated with the TCRBV protein expression using TCRBV-specific monoclonal antibodies. T lymphocytes from four patients with active HCL contained large expansions of particular TCRBV-expressing cells (up to 25% of the CD3+cells; 600 to 700/μL whole blood), which decreased during IFN-α therapy in both patients tested. Finally, restoration of the TCR repertoire matched normalization of the functional immune repertoire as measured by proliferative, helper, and cytotoxic T-lymphocyte precursor frequencies against major histocompatibility complex–unrelated individuals. In conclusion, oligoclonal bands of TCRBV gene families found by RT-PCR correspond with a dramatic increase in circulating T lymphocytes expressing the same TCRBV family. Moreover, IFN-α can restore the skewed T-cell repertoire and suppress persistent T-cell clones upon treatment of the accompanying malignancy.

Description: Therapeutic reactivity of CD20-specific monoclonal antibodies (mAb) or CD19-specific chimeric antigen receptor (CAR)-transduced T cells is exerted by targeting extracellular antigens. In contrast to mAbs and CARs, T cell receptors (TCRs) recognize antigen-derived peptides that are bound to human leukocyte antigen (HLA) molecules on the cell surface. Since HLA molecules constantly sample the entire endogenous proteome of a cell, extracellular and intracellular antigens are presented and can thus be recognized by a TCR. Here, we identified the intracellular transcription factor Bob1 encoded by gene POU2AF1 as a suitable target for immunotherapy. Bob1 is highly expressed in CD19+ B cells, acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL) and multiple myeloma (MM) and is absent in the non-B lineages including CD34+ hematopoietic progenitor cells (HPCs), T cells, fibroblasts, keratinocytes and gastrointestinal tract. Bob1 is localized intracellularly but HLA-presented Bob1-derived peptides are accessible on the cell surface to TCRs and can thus be recognized by T cells. From the HLA-presented ligandome (Mol Cell Proteomics, 2013;12:1829) we identified naturally processed Bob1-derived peptides displayed in HLA-A*0201 (HLA-A2) and in HLA-B*0702 (HLA-B7). Since auto-reactivity towards self-antigens such as Bob1 is prevented by depleting high-avidity T cells recognizing self-antigens in self-HLA, we exploited the immunogenicity of these peptides presented in allogeneic HLA. From a HLA-A2/B7-negative healthy individual we isolated T cell clone 4G11 demonstrating high sensitivity and specificity for Bob1-derived peptide Bob144 presented in HLA-B7. Bob1-dependent recognition was demonstrated by transduction of Bob1 into cell lines that otherwise lack Bob1 expression. No harmful toxicities of clone 4G11 were observed against a wide panel of Bob1-negative stimulator cells including HLA-B7-positive CD34+ HPCs, T cells, monocytes, immature and mature dendritic cells, and fibroblasts even under simulated inflamed conditions. Furthermore, stringent HLA-B7-restricted recognition was observed for clone 4G11 when tested against a stimulator panel expressing a wide range of common and rare HLA class I and II molecules. Clone 4G11 demonstrated clinical applicability by efficiently recognizing HLA-B7+ primary ALL, CLL and MCL. Furthermore, reproducible strong recognition of purified primary HLA-B7+ MM could be demonstrated. Therefore, the TCR of clone 4G11 may be used for immunotherapy by administering TCR-transduced T cells to patients suffering from B cell malignancies including multiple myeloma. Retroviral gene transfer of TCR 4G11 led to efficient cell surface expression demonstrated by binding of TCR-transduced CD8+ T cells to pMHC-tetramer composed of peptide Bob144 bound to HLA-B7. TCR-modified CD8+ T cells strongly recognized Bob1-expressing HLA-B7+ multiple myeloma cell lines U266 and UM9, and ALL cell lines. TCR-modified T cells efficiently lysed HLA-B7+ primary ALL, CLL and MCL at very low effector-to-target ratios. In addition, highly purified primary multiple myeloma samples were also readily lysed. Furthermore, TCR-transduced T cells strongly proliferated in an antigen-specific manner when stimulated with primary malignant cell samples including ALL, CLL, and MCL or MM cell lines. As expected, TCR-transduced T cells also lysed autologous primary and CD40L-stimulated B cells since these targets cells also express Bob1. In contrast, no lysis of Bob1-negative autologous primary and activated T cells, or monocytes was observed when co-cultured with TCR-transduced T cells. In summary, we identified the intracellular transcription factor Bob1 encoded by gene POU2AF1 as a suitable target for TCR-based immunotherapies of B cell malignancies. Bob1-specific T cell clone 4G11 efficiently recognized primary B cell leukemia and multiple myeloma. Gene transfer of TCR of clone 4G11 installed Bob1-reactivity and specificity onto recipient T cells shown here by cytolytic capacity and proliferation upon antigen encounter. TCR gene transfer approaches using this Bob1-specific TCR can bring novel treatment modalities and possibly curative therapy to patients with B cell malignancies including multiple myeloma. Disclosures No relevant conflicts of interest to declare.

Description: HLA mismatched Stem Cell Transplantation (SCT) can be performed in patients with leukemia if no HLA identical donor can be found. Although HLA class I is expressed by almost all recipient cells and the frequency of allo-HLA reactive T-cells is high in all normal donors, the risk of GVHD after single locus mismatched SCT is comparable to that after HLA identical SCT. Thus, the occurrence of GVHD may not simply be explained by recognition of the mismatched HLA by allo-HLA reactive T-cells. Therefore, we characterized in detail the nature of the allo-immune response in an HLA mismatched setting. A patient with acute myeloid leukemia was treated with T-cell depleted SCT from a sibling donor who was HLA identical except for an HLA-A2 crossover. 6 Months after SCT, Donor Lymphocyte Infusion (DLI) of 2.5*10e6 CD3+ T-cells/kg was given for mixed chimerism caused by persistence of patient T-cells. No clinical response and no GVHD developed. 12 months after SCT the leukemia relapsed with 9% blasts in bone marrow, and a second DLI of 7.5*10e6 CD3+ T-cells/kg was given. The patient died of grade IV GVHD 5 weeks after DLI. durign the GVHD flow cytometry of PBMC’s showed conversion of patient to donor type T-cells. 80% Of the CD8 and 40% of the CD4 T-cells were activated, as determined by co-expression of HLA-DR. These activated T-cells were single cell sorted, non-specifically expanded, and tested for alloreactivity using cytotoxicity and cytokine production assays. 46 Out of 56 isolated CD8 clones and 7 out of 88 CD4 clones recognized patient but not donor target cells, indicating that at the time of the GVHD almost 70% of circulating T-cells were alloreactive. The response was highly polyclonal as shown by usage of at least 13 different TCR Vβs by the CD8 clones, and 6 by the CD4 clones. HLA restriction of the clones was tested with HLA blocking antibodies, a panel of HLA-typed target cells and donor EBV-LCL transduced with HLA-A2. All alloreactive CD8 clones were HLA-A2 specific. To further characterize the specificity, CD8 clones were tested against T2 cells loaded with HPLC fractions of peptides eluted from HLA-A2. Some CD8 clones recognized HLA-A2 with all different HPLC fractions, indicating peptide-independent recognition. Other clones recognized one fraction indicating peptide specificity, or several fractions indicating “promiscuous” peptide recognition. The CD4 clones were HLA-DR1 restricted and recognized donor EBV-LCL transduced with HLA-A2, indicating that the peptide recognized in HLA-DR1 was derived from the mismatched HLA-A2 molecule. Therefore, CD4 clones were tested against different peptides covering the whole HLA-A2 sequence. All clones recognized epitope 101–122 derived from a hyper variable region of HLA-A2. These results indicate that the GVHD in this HLA-A2 mismatched transplantation was caused by a combined highly polyclonal CD8 response directed against the HLA-A2 molecule and a CD4 response recognizing an HLA-A2 derived peptide presented by HLA class II. We speculate that the absence of an immune response observed after the first DLI despite high frequency of allo-HLA reactive T-cells, indicates that CD8 anti-HLA-A2 T-cells are insufficient to cause severe GVHD. We hypothesize that the rise of HLA-DR expressing leukemic blasts presenting HLA-A2 derived peptide in HLA class II triggered the CD4 response which was necessary to initiate the CD8 alloresponse resulting in this clinical outcome.

Description: The genetic engineering of T lymphocytes is an attractive strategy to specifically redirect T cell immunity towards viral infections and malignancies. Transfer of virus- or tumor-specific TCRs has demonstrated to endow T cells with redirected antigen specificity. We demonstrated redirected anti-leukemic reactivity of CMV specific T cells using gene transfer of minor histocompatibility antigen HA-2 specific TCRs. The HA-2-TCR-modified T cells exerted high cytolytic activity against HA-2 expressing target cells, including leukemic cells, and not against target cells negative for the HA-2 mHag. After cloning of the TCR-transferred T cells, we demonstrated that the HA-2-TCR cell surface expression, measured by HA-2-tetramer staining, was variable on the transduced T cell clones, and that the cytolytic capacity of the T cells correlated with the level of HA-2-TCR expression. Since we could demonstrate that this variation in HA-2-TCR expression was not due to differences in transgene expression, we investigated whether the endogenous TCRs influenced the expression of the introduced TCR. CMV-A2 specific T cells were isolated from peripheral blood and transduced with the HA-2-TCR. In control transduced CMV specific T cells we observed 5 different high affinity CMV specific TCRs. CMV specific T cells transduced with the HA-2-TCR that expressed predominantly the HA-2-TCR, expressed only one of these types of CMV-TCR, and in CMV specific T cells with low HA-2-TCR expression two different types of CMV-TCRs were found. These data indicated that the level of expression of the introduced TCR is strongly influenced by the endogenous TCR. To investigate whether this was due to differences in promotor activity of the endogenous and retrovirally introduced TCR, the three CMV-TCRs were characterized and transferred into unselected peripheral T cells. T cells transferred with the weak competitior CMV-TCR that was strongly downregulated in CMV specific T cells by introduction of the HA-2-TCR, showed low CMV specific cytotoxicity and no tetramer staining. In contrast, T cells transferred with the strong competitor CMV-TCR that was modestly downregulated in CMV specific T cells by introduction of the HA-2-TCR, revealed strong CMV specific cytotoxic activity and tetramer staining. These data demonstrate that the introduced and endogenous TCRs compete for cell surface expression, and that this competition is dependent on characteristics of the different TCRs and independent of whether the TCR is retrovirally introduced or naturally expressed. To investigate whether the cell surface expression of the different TCRs was determined by preferential pairing properties of the individual TCR chains, TCR α and β deficient Jurkat 76 cells were transduced with the three CMV-specific TCRs or with chimeric TCRs consisting of the TCR α chain of one TCR with the TCR β chain of another TCR. TCRαβ membrane expression revealed that TCRs with a strong competitor phenotype expressed higher levels of TCRαβ than the TCR that was a weak competitor. TCRαβ expression of Jurkat cells transduced with chimeric TCRs indicated that the expression level of the different TCRs was determined by the pairing properties of the individual TCR α and β chains and not by differences in protein expression. In conclusion these data demonstrated that introduced and endogenous TCRs compete for cell surface expression in favor of the TCR that has the highest intrinsic pairing properties.

Description: T cells recognizing solid tumors and Hodgkin's lymphoma can be released from anergy by immune checkpoint inhibitors. Its therapeutic success seems to be associated with the abundance of neoepitopes within the tumor mutanome. In neoplastic B cells, VDJ recombination and somatic hypermutation (SHM) generate unique immunoglobulin (Ig) peptide sequences that predictably contribute to the lymphoma mutanome. Efficient presentation of a neoepitope by an individual's HLA complex is an essential requirement for neoepitope-directed T-cell immunity. Presentation of Ig-derived peptides in HLA class I has hitherto been demonstrated only indirectly, and there is as yet no direct evidence for presentation of Ig-derived neoepitopes of primary human lymphoma cells. We analyzed HLA-presented Ig-derived peptides in 9 clonal B-cell populations: 3 chronic lymphocytic leukemias (CLL), 1 hairy cell leukemia, 1 follicular lymphoma (FL), 3 EBV-transformed lymphoblastoid cell lines, and the U299 myeloma cell line. Full-length B-cell receptor (BCR) VDJ and VJ sequences were obtained by unbiased ARTISAN PCR and Pacific Biosciences next generation sequencing. 1067 unique Ig-derived nonamers were predicted to bind the HLA class I alleles expressed by the respective B-cell clone by the NetCTLpan bioinformatics tool. 650 candidate epitopes (60.9%) were derived from constant (C) regions; 417 (39.1%) from variable (V) regions. 146 predicted peptides from V regions (35.0%) contained at least one amino acid change due to SHM or at least one amino acid from CDR3 and were therefore considered potential neoepitopes. To investigate experimentally which epitopes are processed intracellularly and presented by HLA class I, HLA class I-peptide complexes were immunoaffinity purified by the monoclonal antibody W6.23 and analyzed by liquid chromatography and tandem mass spectrometry. In total, 53,663 unique peptides (8-15mers) were identified by Uniprot matching with a Mascot Ion Score of 〉20. HLA ligandome sizes of individual B-cell populations ranged from 600 to 14,091 unique peptides per case. Within any HLA ligandome, 6 to 81 peptides were annotated as BCR-derived epitopes if they matched the individual BCR sequences. Of the total of 276 eluted BCR peptides, 203 (73.5%) where derived from C regions and 73 (26.5%) from V regions. 25 eluted peptides (range 0-10 per case) were derived from CDR3 regions or contained SHM-induced amino acid changes, thus fulfilling the definition of neoepitopes. Neoepitopes were detected in all cases with more than 109 cells as input material. Up to date, 4 neoepitopes have been synthesized and their mass spectra confirmed. Confirmation of all remaining neoepitopes is ongoing. No BCR-derived neoepitopes could be detected in an IGHV-unmutated CLL and the FL. These two cases had the lowest number of input cells and small overall ligandome sizes of only 600 and 1015 unique UniProt-matched peptides, respectively. We demonstrate for the first time that primary neoplastic B cells process and present BCR-derived neoepitopes in the context of HLA class I. Despite their origin from only two polypeptides, BCR epitopes and neoepitopes represent app. 0.5% and 0.05% of the total HLA class I ligandome, respectively. The challenging identification of BCR neoepitopes was possible by unbiased identification of BCR transcripts combined with next generation sequencing and targeted search for HLA class I-bound peptides. Matching fragmentation patterns of native and synthetic peptides suggest high specificity of this strategy. With respect to sensitivity, the ability to identify low frequency peptides appears to be strongly dependent on the amount of input cells. Our data close a gap in the mechanism underlying the evidence that highly immunogenic formulations of an idiotype vaccine are able to induce MHC class I-restricted cytotoxic T-cell responses. While phase III trials of idiotype vaccination aiming to induce anti-Ig antibodies failed to demonstrate convincing prolongation of clinical remissions achieved by chemotherapy, our data lend support for exploring idiotype-specific T-cell immunity against B-cell lymphomas. With recent advances in peptide synthesis, adjuvant formulations, and the availability of check point inhibitors to surpass regulatory activity, active immunotherapy targeting the lymphoma idiotype may regain appeal as truly personalized immune therapy. Disclosures No relevant conflicts of interest to declare.

Description: Abstract 4136 Purpose: Allogeneic stem cell transplantation (allo-SCT) can induce remission in patients with hematological malignancies due to graft-versus-tumor (GVT) responses. This, however, is often accompanied by graft-versus-host disease (GVHD). Both the GVT effect and GVHD are mediated by minor histocompatibility antigen (MiHA)-specific T cells recognizing peptide products from polymorphic genes that differ between recipient and donor. Here, we evaluated whether MiHA mismatches are associated with clinical outcome after partial T cell depleted allo-SCT. Patients and Methods: We retrospectively analyzed the impact of MiHA mismatches in a cohort of 327 patients who received a partially T cell-depleted allo-SCT because of a hematological malignancy. MiHA allele genotyping was performed by fluorescence-based competitive allele-specific PCR. Subsequently, a multivariable statistical analysis of immunogenic MiHA disparity rates and association with clinical outcome was performed. In addition, development of MiHA-specific T cell responses was assessed by dual-color tetramer staining. Results: Statistical analysis revealed that an autosomal MiHA disparity on DNA level associates with increased relapse-free survival in sibling transplants, especially in patients transplanted for multiple myeloma. In addition, mismatches for the ubiquitous Y chromosome-encoded MiHA resulted in more acute GVHD (grade 3–4), while other MiHA mismatches, either ubiquitous or restricted to hematopoietic cells, were not associated with GVHD. Finally, we demonstrated considerable differences between MiHA in the capability to induce in vivo T cell responses post-transplantation. Conclusion: These data support that autosomal MiHA contribute to the induction of GVT immunity providing a rationale for MiHA-based post-transplantation immunotherapy to prevent and treat persistent and recurrent cancer following allo-SCT. Disclosures: No relevant conflicts of interest to declare.

Description: Patients with the B-cell malignancy hairy cell leukemia (HCL) exhibit a skewed T-cell repertoire with oligoclonal expression or absence of many members of the T-cell receptor (TCR) BV gene families. To evaluate whether interferon-α (IFN-α) therapy would not only restore normal hematopoiesis, but also the abnormal T-cell repertoire, we studied T lymphocytes from a cohort of HCL patients treated by IFN-α in the past, at initiation, and at several intervals up to 6 years of IFN-α treatment. The junctional regions from 22 TCRBV gene families were analyzed after polymerase chain reaction amplification of cDNA (RT-PCR) using family specific primers. In all seven patients improvement of the skewed T-cell repertoire was not seen until 2 years of treatment. It consisted of disappearance of oligoclonal subpopulations and (polyclonal) reappearance of absent TCRBV gene families. The RT-PCR results were correlated with the TCRBV protein expression using TCRBV-specific monoclonal antibodies. T lymphocytes from four patients with active HCL contained large expansions of particular TCRBV-expressing cells (up to 25% of the CD3+cells; 600 to 700/μL whole blood), which decreased during IFN-α therapy in both patients tested. Finally, restoration of the TCR repertoire matched normalization of the functional immune repertoire as measured by proliferative, helper, and cytotoxic T-lymphocyte precursor frequencies against major histocompatibility complex–unrelated individuals. In conclusion, oligoclonal bands of TCRBV gene families found by RT-PCR correspond with a dramatic increase in circulating T lymphocytes expressing the same TCRBV family. Moreover, IFN-α can restore the skewed T-cell repertoire and suppress persistent T-cell clones upon treatment of the accompanying malignancy.