Allorecognition of purified major histocompatibility complex glycoproteins by cytotoxic T lymphocytes

Optimizing the Clinical Impact of CAR-T Cell Therapy in B-Cell Acute Lymphoblastic Leukemia: Looking Back While Moving Forward

Chimeric antigen receptor T-cell (CAR-T) therapy has been successful in creating extraordinary clinical outcomes in the treatment of hematologic malignancies including relapsed or refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL). With several FDA approvals, CAR-T therapy is recognized as an alternative treatment option for particular patients with certain conditions of B-ALL, diffuse large B-cell lymphoma, mantle cell lymphoma, follicular lymphoma, or multiple myeloma. However, CAR-T therapy for B-ALL can be surrounded by challenges such as various adverse events including the life-threatening cytokine release syndrome (CRS) and neurotoxicity, B-cell aplasia-associated hypogammaglobulinemia and agammaglobulinemia, and the alloreactivity of allogeneic CAR-Ts. Furthermore, recent advances such as improvements in media design, the reduction of ex vivo culturing duration, and other phenotype-determining factors can still create room for a more effective CAR-T therapy in R/R B-ALL. Herein, we review preclinical and clinical strategies with a focus on novel studies aiming to address the mentioned hurdles and stepping further towards a milestone in CAR-T therapy of B-ALL.

Strategies for Dodging the Obstacles in CAR T Cell Therapy

Chimeric antigen receptor (CAR) T cell therapy has offered cancer patients a new alternative therapeutic choice in recent years. This novel type of therapy holds tremendous promise for the treatment of various hematologic malignancies including B-cell acute lymphoblastic leukemia (B-ALL) and lymphoma. However, CAR T cell therapy has experienced its ups and downs in terms of toxicities and efficacy shortcomings. Adverse events such as cytokine release syndrome (CRS), neurotoxicity, graft rejection, on-target off-tumor toxicities, and tumor relapse have tied the rescuing hands of CAR T cell therapies. Moreover, in the case of solid tumor treatment, CAR T cell therapies have not yielded encouraging results mainly due to challenges such as the formidable network of the tumor microenvironments (TME) that operates in a suppressive fashion resulting in CAR T cell dysfunction. In this review, we tend to shine a light on emerging strategies and solutions for addressing the mentioned barriers. These solutions might dramatically help shorten the gap between a successful clinical outcome and the hope for it.

In vivo apoptosis of CD8(+) lymphocytes in acute myeloid leukemia patients: involvement of soluble HLA-I and Fas ligand

In this study, we show that high serum levels of soluble human leukocyte antigens (HLA) class I molecules (sHLA-I, range: 0.7-1.7 micro g/ml) and soluble Fas ligand (FasL, range: 0.4-1.9 ng/ml) are detected in patients with acute myeloid leukemia (AML) at diagnosis, compared with healthy donors (HD) (sHLA-I, range: 0.1-0.6 micro g/ml; sFasL, range: 0.1-0.4 ng/ml). Patients' sera were able to induce transcription and secretion of FasL in CD8(+) T cells, followed by apoptosis in vitro; this apoptosis was inhibited by anti-HLA-I-specific monoclonal antibodies, suggesting that sHLA-I is responsible for cell death. These findings closely relate to the in vivo upregulation of FasL transcription observed in peripheral blood (PB) lymphocytes from AML patients; in the same cells, mRNA for the antiapoptotic proteins Bcl-2 and Bcl-x(L) was downregulated. Interestingly, caspase-8 and caspase-3, both downstream mediators of death receptor-induced apoptosis, were activated in CD8(+) cells of AML patients; one-third of these cells were already apoptotic in vivo, at variance with lymphocytes of HD. These data strongly suggest that in AML, increased levels of sHLA-I molecules may contribute to the elimination of potentially anti-tumor effector cells through a FasL/Fas interaction.

Sensitization of tumour cells to lysis by virus-specific CTL using antibody-targeted MHC class I/peptide complexes

A number of cell surface molecules with specificity to tumour cells have been identified and monoclonal antibodies (mAb) to some of these antigens have been used for targeting tumour cells in vivo. We have sought to link the powerful effector mechanisms of cytotoxic T-cells with the specificity of mAb, by targeting recombinant HLA class I molecules to tumour cells using an antibody delivery system. Soluble recombinant MHC class I/peptide complexes including HLA-A2.1 refolded around an immunodominant peptide from the HIV gag protein (HLA-A2/gag) were synthesized, and the stability of these complexes at 37 degrees C was confirmed by enzyme-linked immunosorbent assay using a conformation-specific antibody. MHC class I-negative lymphoma cells (Daudi) were labelled with a biotinylated mAb specific for a cell surface protein (anti-CD20) then linked to soluble biotinylated HLA-A2/gag complexes using an avidin bridge. Flow cytometry revealed strong labelling of lymphoma cells with HLA-A2/gag complexes (80-fold increase in mean channel fluorescence). CTL specific for HLA-A2/gag efficiently lysed complex-targeted cells, while control CTL (specific for an HLA-A2.1-restricted epitope of melan-A) did not. Similarly, SK-mel-29 melanoma cells were also efficiently lysed by HLA-A2/gag-specific CTL when HLA-A2/gag complexes were linked to their surface via the HMW-MAA specific anti-melanoma antibody 225.28s. With further consideration to the in vivo stability of the MHC class I/peptide complexes, this system could prove a new strategy for the immunological therapy of cancer.

Purified truncated recombinant HLA-B7 molecules abrogate cell function in alloreactive cytotoxic T lymphocytes by apoptosis induction

BACKGROUND

Soluble MHC class I molecules are ubiquitous in human body fluids, including serum, urine, sweat, and cerebrospinal fluid. However, their biological function has remained unresolved. Membrane-derived human soluble MHC molecules (soluble human leukocyte antigen; sHLA) have been shown to induce apoptosis in alloreactive cytotoxic T lymphocytes (CTL). Here we report the efficacy of recombinant soluble HLA-B7 (rsHLA-B7) to modulate T-cell function.

METHODS

Primers of HLA-B7 were designed to allow amplification of a cDNA lacking the transmembrane and cytoplasmic domains yielding a truncated gene. rsHLA-B7 molecules were expressed in the human myeloma cell line 721.221 and purified by affinity chromatography using the BB7.7 mouse monoclonal antibody. CTL were generated from peripheral blood lymphocytes derived from healthy blood donors by stimulation with irradiated Epstein Barr virus-transformed HLA-B7-positive B cells. CTL were preincubated with rsHLA-B7, and cytotoxicity and apoptosis were tested according to standard procedure.

RESULTS

A total of 2 x 10(6) cells/ml secreted 10 microg/ml rsHLA-B7 as determined by a conformation-dependent ELISA, suggesting that rsHLA-B7 do not require the transmembrane and cytoplasmic regions for proper folding. After purification by affinity chromatography, rsHLA-B7 induced apoptosis in anti-HLA-B7 CTL, but not in anti-HLA-A2-specific, CTL. As a consequence, allorecognition of target cells by the CTL was significantly blocked.

CONCLUSION

Recombinant sHLA are sufficient binding cues for T cells, which efficiently induce apoptosis and block allorecognition of target cells by CTL. Thus, recombinant sHLA molecules may become a valuable new modality for specific immunological therapeutic intervention.

Soluble HLA class I molecules: biological significance and clinical implications

Soluble class I human leukocyte antigens (sHLAs) have been detected in serum, sweat, lymphatic fluid, urine and cerebrospinal fluid. Their biological function has, however, remained a puzzle. The physiological concentration of sHLA varies more than tenfold depending on the phenotype of the individual, and is significantly upregulated in various diseases and during inflammation. This suggests that sHLAs might serve as a marker of pathological changes. Recent experiments have shown that, in vitro, sHLAs can modulate T-cell reactivity and induce cell-activated apoptosis, implicating sHLAs in the induction and maintenance of peripheral tolerance. Therefore, sHLAs have the therapeutic potential to induce tolerance to transplants.

T cells receptor beta-chain repertoires are nonrandomly selected in responses to HLA-DR1

T cell receptor (TCR) beta-chain usage by HLA-DR1 alloreactive T cell lines was examined to determine whether common TCR gene segments were used preferentially. We have demonstrated previously that a DR1-specific, human renal allograft-derived T cell line and replicate, anti-DR1 mixed lymphocyte reactions (MLR), established from an unrelated responder/stimulator pair, were selected for T cells expressing V beta 8. In this report V beta 8+ beta-chains from these T cell lines were sequenced to assess clonality and determine the contribution made by the beta-chain junctional regions. All 11 V beta 8+ cDNA clones sequenced from the allograft-derived T cell lines used J beta 2.7 and C beta 2 and had identical junctions, indicating the presence of a predominant V beta 8+ clone. All seven V beta 8+ sequences from the first anti-DR1 MLR and eight of the nine fron the second also used J beta 2.7 and C beta 2 were identical to one another, indicating that a common V beta 8+ clone was selected in these replicate cultures. The sequences of the predominant V beta 8+ beta-chains from the allograft-derived T-cell line and the MLR differed by only 10 nucleotides and four amino acids at the VDJ beta junction. To determine the reproducibility of TCR V beta selection in responses to DR1, additional MLR were established by pairing three different DR1+ stimulators with the same responder. The TCR repertoires of the resulting DR1-specific cell lines were examined. A preference was seen for utilization for certain homologous TCR V beta segments. The data suggest that particular TCR V beta or V beta/J beta combinations may be selected in alloresponses as evidenced either utilization of highly similar beta-chains or homologous V beta segments.

Interaction of papain-digested HLA class I molecules with human alloreactive cytotoxic T lymphocytes (CTL)

Acute immunological rejection events of transplanted allogeneic organs are strongly dependent on T cell reactivity against foreign MHC products. The recognition requirements of alloreactive cytotoxic T cells are of particular interest for finding approaches to modulating alloreactivity. The role of the allogeneic MHC molecule itself and/or an associated peptide in the interaction with the T cell receptor is still, however, unclear. Our studies have focused on the interactions of papain-digested HLA class I molecules with alloreactive CD8+ CTL. These polypeptides, consisting of the polymorphic alpha 1 and alpha 2 and the monomorphic alpha 3 domains, were used in both soluble and immobilized form to study their functional effects on anti-HLA-A2 reactive CTL. Purified polypeptides were of molecular mass 32-34 kD. HLA-A2 polypeptides (0.55 micrograms/ml) in soluble form induced half-maximal reduction of CTL cytotoxicity. These concentrations were quantitatively comparable to the effective doses of intact HLA class I molecules, which contain the hydrophobic transmembrane domain and the intracytoplasmic tail. In addition, specific activation requirements of these CTL were investigated in a serine esterase release assay. Maximal degranulation was observed after 2 h of antigen contact. Purified HLA class I molecules allospecifically activated the anti-HLA-A2 CTL to degranulate serine esterase, when immobilized on plastic microtitre plates. Thus, polypeptides containing the polymorphic alpha 1 and alpha 2 domains of human class I molecules potentially modulate the cytotoxic T cell response. This might have implications for the reduction or prevention of allograft rejection in recipients of foreign organs.

HLA-A2-peptide complexes: refolding and crystallization of molecules expressed in Escherichia coli and complexed with single antigenic peptides

The two subunits of the human class I histocompatibility antigen (HLA)-A2 have been expressed at high levels (20-30 mg/liter) as insoluble aggregates in bacterial cells. The aggregates were dissolved in 8 M urea and then refolded to form an HLA-A2-peptide complex by removal of urea in the presence of an antigenic peptide. Two peptides from the matrix protein and nucleoprotein of influenza virus are known to bind to HLA-A2, and both support the refolding of the recombinant HLA-A2 molecule. An additional peptide, a nonamer from the gp120 envelope protein of human immunodeficiency virus type 1, also supported refolding. Yields of purified recombinant HLA-A2 are 10-15%. In the absence of an HLA-A2-restricted peptide, a stable HLA-A2 complex was not formed. Monoclonal antibodies known to bind to native HLA-A2 also bound to the recombinant HLA-A2-peptide complex. Three purified HLA-A2-peptide complexes refolded from bacterially produced protein aggregates crystallize under the identical conditions as HLA-A2 purified from human lymphoblastoid cells. Crystals of the recombinant HLA-A2 molecule in complex with the influenza matrix nonamer peptide, Mp(58-66), diffract to greater than 1.5-A resolution.

Alloreactive cytotoxic T lymphocytes recognize epitopes determined by both the alpha helices and beta sheets of the class I peptide binding site

A chimeric class I glycoprotein was created to investigate the functional contribution of the alpha helices and the beta-pleated sheets in forming the antigen recognition site (ARS) of antigen-presenting molecules. This novel molecule was generated by replacing the DNA sequences encoding the alpha helices of the Ld gene with the corresponding sequences from the Kb gene. Serologic analysis of transfected L cells that expressed the chimeric molecule (Kb alpha Ld beta) revealed that the engineered class I glycoprotein retains two conformational epitopes associated with the alpha helices of Kb, as defined by monoclonal antibodies K10.56 and 28-13-3. These results demonstrate that the alpha helices of Kb can associate with the beta-pleated sheets of Ld to form a stable structure, which is expressed on the cell surface. To address the role of the alpha helices of the ARS in determining T cell crossreactivity, alloreactive cytotoxic T lymphocytes (CTL) were used to analyze L cells expressing Kb alpha Ld beta. CTL raised against Kb or Ld as alloantigens showed little, if any, ability to lyse L cells expressing Kb alpha Ld beta. Thus, alloreactive CTL did not recognize structures determined by the alpha helices alone or by the beta sheets of the ARS alone. However, bulk and cloned alloreactive CTL that were generated against the mutant Kb glycoprotein Kbm8 reacted strongly with Kb alpha Ld beta. In addition to the Kb alpha helices, the Kbm8 ARS shares a single polymorphic amino acid at position 24 with Kb alpha Ld beta. Amino acid 24 is located on the beta 2 strand that forms part of the floor of the ARS and has been identified as a component of pocket B in the HLA class I ARS. The substitution of Glu to Ser at this position was shown previously to be the central determinant of the Kbm8 mutant alloantigenicity. The functional significance of this position in determining crossreactivity between bm8 and Kb alpha Ld beta identifies pocket B as a strong anchor for allogenic self-peptides. These findings demonstrate that determinants recognized by CTL on class I alloantigens are formed by interactions involving both the alpha helices and beta sheets of the ARS. These interactions are best explained by the influence of the alpha helices and beta sheets on the peptide-binding properties of these antigen-presenting molecules.

Structural and functional characterization of HLA-DR molecules circulating in the serum

HLA molecules are highly polymorphic glycoproteins with a single binding site for immunogenic peptides. The complex formed by HLA-DR molecules and peptides is the entity specifically recognized by the antigen receptor of CD4+ helper T lymphocytes. This biological function has been linked to the constitutive cell surface expression of HLA molecules on antigen-presenting cells which provide immunogenic peptides through denaturation or fragmentation of antigen. Here, we report that HLA-DR molecules can be identified in a soluble form circulating in the serum. Biochemical characterization and recognition of such soluble HLA-DR molecules by a monoclonal antibody reactive to conformational determinants on the alpha-beta HLA complex indicates that they are circulating as intact heterodimers. Upon activation, human T cells and B cells release soluble HLA-DR molecules. The expression and secretion of HLA-DR by T lymphocytes is greatly enhanced in the presence of macrophages indicating that the production of HLA-DR molecules might be regulated and controlled through factors derived from antigen-presenting cells. An alloreactive human T cell clone can be stimulated to proliferate in response to soluble HLA-DR molecules purified and enriched from serum suggesting that the soluble form of the natural ligand of the T cell receptor might have immunoregulatory functions.

Cytotoxic T lymphocytes recognize a reconstituted class I histocompatibility antigen (HLA-A2) as an allogeneic target molecule

Recent findings suggest that peptide fragments of newly synthesized proteins may associate intracellularly with nascent chains of class I histocompatibility antigens (termed MHC-I proteins because they are encoded by genes of the major histocompatibility complex) and that these peptide adducts may be required for the folding or stability and perhaps even the transport of these proteins to the cell surface. To determine whether these proteins can be reconstituted from their separated subunits into ostensibly native molecules in the absence of added peptides, we denatured a purified human MHC-I protein (HLA-A2) with 4 M NaSCN, separated its heavy (alpha) and light (beta 2-microglobulin) chains by gel filtration, and then mixed them in the presence of a 3-fold molar excess of beta 2-microglobulin and absence of added peptides. The reconstituted protein, recovered in 10% yield, was indistinguishable from native A2 in its reactivity with a monoclonal antibody (BB7.7) and its ability to specifically activate A2-specific CD8+ T cells. Inasmuch as the reconstituted A2 contained no detectable peptide adducts (we estimate less than 1 per 100 on a molar basis, assuming peptides of 2-5 kDa), the results suggest that peptide-free A2 can be recognized by CD8+ T cells and that peptide adducts are not essential for the MHC-I protein to maintain an ostensibly native structure.

Mapping of the full profile of T cell allorecognition regions on HLA-DR2 beta subunit

Alloreactive human T cell lines were developed by repeated in vitro restimulation with alternate allogeneic cells of similar DR2 serotype, but differing at other HLA loci. These polyclonal T cells recognized DR specificities in common with all DR2 serotypes. The allorecognition profiles of DR2 beta by the T cell lines were mapped by consecutive uniform-sized overlapping peptides encompassing the entire extracellular and intracellular regions of the DR beta subunit. It was found that the T cell allorecognition sites are focused around the first and third polymorphic regions of the N-terminal domain, and interestingly, include conserved areas in the DR beta second domain as well as the intracellular segment beta 222-237. These findings have important implications for allorecognition. Comparison with the previous analysis of antibody-binding regions of DR2 beta revealed some similarities and differences in the antibody and T cell alloresponses to this histocompatibility antigen.