Simultaneous monitoring of binding to and activation of tumor-specific T lymphocytes by peptide-MHC

T-cells "à la CAR-T(e)" - Genetically engineering T-cell response against cancer

The last decade will be remembered as the dawn of the immunotherapy era during which we have witnessed the approval by regulatory agencies of genetically engineered CAR T-cells and of checkpoint inhibitors for cancer treatment. Understandably, T-lymphocytes represent the essential player in these approaches. These cells can mediate impressive tumor regression in terminally-ill cancer patients. Moreover, they are amenable to genetic engineering to improve their function and specificity. In the present review, we will give an overview of the most recent developments in the field of T-cell genetic engineering including TCR-gene transfer and CAR T-cells strategies. We will also elaborate on the development of other types of genetic modifications to enhance their anti-tumor immune response such as the use of co-stimulatory chimeric receptors (CCRs) and unconventional CARs built on non-antibody molecules. Finally, we will discuss recent advances in genome editing and synthetic biology applied to T-cell engineering and comment on the next challenges ahead.

Long-lived memory CD8+ T cells are programmed by prolonged antigen exposure and low levels of cellular activation

CD8+ T cells play a crucial role in controlling intracellular pathogens. The level of memory CD8+ T cells developing after vaccination or infection influences the degree of T cell-mediated protection after secondary infection. We used defined animal models and infections/immunizations by replicating or non-replicating antigens to define on a molecular and cellular level in vivo the parameters that identify and shape long-lived CD8+ T cell memory. We show that the timing of antigen exposure during vaccination is key for the induction of long-lived T cell memory. Brief antigen exposure induced high numbers of effector cells but limited development of long-lived CD8+ memory T cells. In contrast, prolonged antigen exposure for up to 9 days induced similar numbers of effector T cells but additionally resulted in high levels of memory CD8+ T cells. Unexpectedly CD127 (IL-7Ralpha) expression on CD8+ T cells during the acute priming phase was a necessary but not sufficient requirement for entering the pool of long-lived antigen-independent memory CD8+ T cells. However, we provide strong evidence for the interpretation that programming of long-lived memory T cells was driven by low levels of transcription factor eomesodermin and protease inhibitor Spi2A as well as reduced phosphorylation of c-JUN.

Tetramer-blocking assay for defining antigen-specific cytotoxic T lymphocytes using peptide-MHC tetramer

Peptide-MHC tetramers have been engineered to allow accurate detection of antigen-specific cytotoxic C lymphocytes (CTL) by flow cytometry. Here, we propose a novel use for peptide-MHC tetramers in the specific and sensitive analysis of the cytotoxic function of antigen-specific CTL by blocking MHC-restricted antigen-specific cytotoxicity. We found that pretreatment of ovalbumin (OVA)-specific CD8(+) CTL (OT-1 CTL), derived from OT-1 T-cell receptor (TCR)-transgenic mice, with OVA(257-264) peptide-H-2K(b) tetramer caused a marked inhibition of the cytotoxicity against OVA-expressing EG-7 tumor cells. OVA(257-264) peptide-H-2K(b) tetramer did not block the cytotoxicity mediated by 2C mouse (H-2(b))-derived CD8(+) CTL, which recognize allo (H-2L(d)) antigens. Moreover, OT-I CTL activity was not inhibited by an irrelevant HBV(208-216) peptide-H-2K(b) tetramer. These results indicate that the blocking of CTL activity with peptide-MHC tetramer was caused by interference with the interaction between the TCR and H-2K(b)-OVA(257-264) peptide complex, but not with the CD8-MHC class I interaction. The blocking activity of OVA(257-264) peptide-H-2K(b) tetramer was reversible because OT-I CTL pretreated with the tetramer recovered their cytotoxicity after culturing with interleukin-2 for 24 h. The same results were also demonstrated in freshly isolated, in vivo-primed OT-1 CTL sorted by the tetramer. These results demonstrate that peptide-MHC tetramer is a useful tool for defining MHC-restricted antigen-specific CTL function. Moreover, our finding implies that the measurement of CTL activity immediately after tetramer-guided sorting is not a suitable method for evaluating the function of in vivo-induced tetramer-positive CTL. We believe that the tetramer-blocking assay presented here will be useful for functionally monitor the induction of MHC-restricted antigen-specific CTL during vaccination therapy against tumor and infectious diseases.

Tetramer analysis of human autoreactive CD4-positive T cells

Self-reactivity is an intrinsic property of the human immune system. Autoreactive T cells derive directly from the developmental requirement for TCR engagement by self-antigens during lymphocyte maturation. The fundamental questions implicating these autoreactive cells in human autoimmunity then, are not "Where do they come from?", but rather "Why do they persist?", "How do they become activated?", and "How are they regulated or deleted?". New technologies, in which peptide-MHC (pMHC) ligands used for T-cell recognition are utilized as soluble fluorescent multimers, now permit the direct visualization of antigen-specific autoreactive T-lymphocytes. By using multimer technology to study self-reactive cells present in autoimmune patients and control individuals, a very broad range of autoreactive potential has been identified.

Tumor-specific Ab-mediated targeting of MHC-peptide complexes induces regression of human tumor xenografts in vivo

A cancer immunotherapy strategy is described herein that combines the advantage of the well established tumor targeting capabilities of high-affinity recombinant fragments of Abs with the known efficient, specific, and potent killing ability of CD8 T lymphocytes directed against highly antigenic MHC-peptide complexes. Structurally, it consists of a previously uncharacterized class of recombinant chimerical molecules created by the genetic fusion of single-chain (sc) Fv Ab fragments, specific for tumor cell surface antigens, to monomeric scHLA-A2 complexes containing immunodominant tumor- or viral-specific peptides. The fusion protein can induce very efficiently tumor cell lysis, regardless of the expression of self peptide-MHC complexes. Moreover, these molecules exhibited very potent antitumor activity in vivo in nude mice bearing preestablished human tumor xenografts. These in vitro and in vivo results suggest that recombinant scFv-MHC-peptide fusion molecules could represent an approach to immunotherapy, bridging Ab and T lymphocyte attack on cancer cells.

Direct Measurement of Antigen Binding Properties of CD1 Proteins Using Fluorescent Lipid Probes

CD1 proteins are antigen-presenting molecules that bind foreign and self-lipids and stimulate specific T cell responses. In the current study, we investigated ligand binding by CD1 proteins by developing a fluorescent probe binding approach using soluble recombinant human CD1 proteins. To increase stability and yield, soluble group 1 CD1 (CD1b and CD1c) and group 2 CD1 (CD1d) proteins were produced as single chain secreted CD1 proteins in which beta2-microglobulin was fused to the N termini of the CD1 heavy chains by a flexible peptide linker sequence. Analysis of ligand binding properties of single chain secreted CD1 proteins by using fluorescent lipid probes indicated significant differences in ligand preference and in pH dependence of binding by group 1 versus group 2 CD1 proteins. Whereas group 1 CD1 isoforms (CD1b and CD1c) show stronger binding of nitrobenzoxadiazole (NBD)-labeled dialkyl-based ligands (phosphatidylcholine, sphingomyelin, and ceramide), group 2 CD1 (CD1d) proteins were stronger binders of small hydrophobic probes such as 1-anilinonaphthalene-8-sulfonic acid and 4,4'-dianilino-1,1'-naphthyl-5,5'-disulfonic acid. Competition studies indicated that binding of fluorescent lipid probes involved association of the probe with the hydrophobic ligand binding groove of CD1 proteins. Analysis of selected alanine substitution mutants of human CD1b known to inhibit antigen presentation showed that NBD-labeled lipid probe binding could be used to distinguish mutations that interfere with ligand binding from those that affect T cell receptor docking. Our findings provide further evidence for the functional specialization of different CD1 isoforms and demonstrate the value of the fluorescent lipid probe binding method for assisting structure-based studies of CD1 function.