IL-2/αIL-2 Complexes Massively Expand Systemic Tumor-Specific T Cells and Enhance Abscopal Responses to Radiation and αPD-1

PURPOSE/OBJECTIVE(S)

Early clinical trials have provided evidence for RT-induced systemic effects in conjunction with αPD1 or IL-2 in metastatic patients, but strong abscopal responses are clinically rare. We investigated the effect of a triple combination of CD122-directed IL-2/αIL-2 complexes (IL-2c), hRT, and αPD1 on tumor-specific CD8+ T cell differentiation and the abscopal effect vs. the respective dual treatments.

MATERIALS/METHODS

Mice bearing bilateral tumors were treated with two fractions of 8 Gy (C51 colon carcinoma model) or 12 Gy (B16 melanoma model); αPD1 was given weekly; IL-2c was given for five consecutive days. Dependence of the therapeutic effect on CD8+ T cells and T cells expressing the chemokine receptor CXCR3 was assessed using depleting or blocking antibodies. Differentiation stages of tumor-specific CD8+ T cells in various compartments were determined flow cytometrically using MHC-I tetramers and appropriate antibodies. Anti-tumoral effects of blood-derived and tumor-derived T cells were assessed in adoptive T cell transfer experiments.

RESULTS

The abscopal effect was significantly stronger in triple-treated mice compared to mice treated with RT/αPD1 (C51 model: p < 0.01; B16 model: p < 0.05), RT/IL-2c (C51 model: p < 0.01; B16 model: p < 0.001) or αPD1/IL-2c (C51 model: p < 0.0001, B16 model: p < 0.01). Triple therapy improved survival and resulted in complete cures of 3/12 mice in the C51 model and 2/12 mice in the B16 model. These anti-tumor effects were associated with dramatic expansion of tumor-specific CD8+ T cells. Undifferentiated stem-like (TCF1+TIM3-PD1+) and effector-like (CD101-TIM3+TCF1-PD1+) but not terminally differentiated (CD101+TIM3+TCF1-PD1+) exhausted cells particularly strongly increased. Moreover, IL-2c induced CXCR3 mainly on non-terminally differentiated CD8+ T cells. Both CD8+ (C51 model: p < 0.0001; B16 model: p < 0.01) and CXCR3+ (C51 model: p < 0.0001) T cells were crucial for the RT-induced abscopal effect upon RT/αPD1/IL-2c treatment. Finally, we found that peripheral blood from triple-treated mice is an effective source of T cells for adoptive T cell transfer.

CONCLUSION

RT/αPD1/IL-2c triple treatment resulted in superior local and systemic expansion of tumor-specific CD8+ T cells with stem- and effector-like phenotypes. Also, IL-2c strongly increased CXCR3+CD8+ T cells that were associated with pronounced abscopal responses in models with an established metastasis resistant to αPD1/IL-2c and only transiently responding to RT/αPD1 or RT/IL-2c. Therefore, such triple combinations appear promising for clinical evaluation in metastatic patients.

Doxorubicin Enhances the Abscopal Effect Depending on Tumor Cell Mitochondrial DNA and cGAS/STING

PURPOSE/OBJECTIVE(S)

Localized radiotherapy (RT) can cause a T cell-mediated abscopal effect on non-irradiated tumor lesions, especially in combination with immune checkpoint blockade (ICB). However, this effect is still clinically rare and improvements are highly desirable. We investigated whether triple combination with a low dose of clinically approved liposomal doxorubicin (Doxil) could augment abscopal responses compared with RT+ICB, Doxil+ICB, or RT+Doxil.

MATERIALS/METHODS

We used Doxil in combination with RT and αPD1 in two tumor models (B16-CD133 melanoma and MC38 colon carcinoma) with mice bearing two tumors, only one of which was irradiated.

RESULTS

Triple therapy with RT, αPD1, and single low-dose Doxil strongly enhanced the RT-induced abscopal effect compared to all double and single treatments in both tumor models (p < 0.05, n = 5-10 mice/group). Complete cures of non-irradiated tumors were mainly observed in triple-treated mice. Triple therapy induced more cross-presenting dendritic cells (DCs) and tumor-specific CD8 T cells than RT/αPD1 and Doxil/αPD1 (p < 0.05, n = 5 mice/group), particularly in non-irradiated tumors. CD8 T cell depletion or implanting STING-deficient tumor cells abolished the abscopal effect. By using inhibitors and knockout cells, we show that doxorubicin/Doxil-induced IFNβ1 markedly depended on the cGAS/STING pathway (p < 0.05) which drives antitumor CD8 T cell responses through cross-presenting DCs. In mitochondrial DNA (mtDNA)-depleted tumor cells, doxorubicin/Doxil induced less IFNβ1 (p < 0.05), the related T cell-recruiting chemokine CXCL10 (p < 0.0001), and ATP (p < 0.0001); coincubation with mtDNA-depleted tumor cells strongly reduced IFNβ1 (p < 0.01) secretion by DCs. Implantation of mtDNA-depleted tumor cells, particularly at the non-irradiated site, substantially diminished the Doxil-enhanced abscopal effect and tumor infiltration by tumor-specific CD8 T cells (p < 0.05).

CONCLUSION

Single low-dose Doxil can substantially enhance the RT-induced abscopal effect, with a strong increase in cross-presenting DCs and CD8 tumor-specific T cells particularly in abscopal tumors compared with RT/αPD1 and Doxil/αPD1. The mtDNA/cGAS/STING/IFN-I axis is important for the immunogenic doxorubicin effects. Our findings may be helpful for the planning of clinical radiochemoimmunotherapy trials in (oligo)metastatic patients.

Comparison of Two Triple Therapy Regimens for Enhancing the Abscopal Effect in Mice

PURPOSE/OBJECTIVE(S)

Localized radiotherapy (RT) can cause a T cell-mediated abscopal effect on non-irradiated tumor lesions, especially in combination with immune checkpoint blockade (ICB). However, this effect is still clinically rare and improvements are highly desirable. We here compared two triple therapy regimens (RT+⍺PD1+⍺CTLA4 vs. RT+⍺PD1+IL2/⍺IL2 complexes (IL2c)) for their efficacy to improve systemic (abscopal) anti-tumor effects.

MATERIALS/METHODS

In mice bearing bilateral subcutaneous C51 colon carcinoma tumors, the primary tumor was irradiated with two fractions of 8 Gy. ⍺PD1 and ⍺CTLA4 were given concomitantly and weekly thereafter; IL2c was given i.p. for 3 consecutive days. Besides tumor size and survival, tumor-specific CD8+ T cells were determined flow cytometrically using MHC-I tetramers and various antibodies. In addition, isolated TILs were cultured with PMA, ionomycin, and Brefeldin A in vitro to assess polyfunctionality (based on cytokine production) of CD8+ and CD4+ TILs.

RESULTS

The abscopal effect was significantly stronger in mice treated with the ⍺CTLA4-containing triple combination (n = 13) than in mice treated with the IL2c-containing triple combination (n = 9) (p<0,05), and compared to mice treated with RT+⍺PD1 (n = 8) (p<0,05) or RT+⍺CTLA4 (n = 10) (p<0,05), respectively. The IL2c triple combination induced the abscopal effect better than RT+⍺PD1 (p<0,05). The ⍺CTLA4 triple therapy improved survival and resulted in complete cures of 8/13 mice. In mice treated with ⍺CTLA4-containing triple therapy, control of the irradiated tumor was partially dependent and that of the non-irradiated tumor was strongly dependent on T cells, primarily CD8 T cells but also CD4 T cells. With ⍺CTLA4 triple treatment (n = 9), the frequency and absolute numbers of polyfunctional TNF-⍺+IFN-γ+ AH-1 tumor-specific CD8+ T cells, IFN-γ+IL2+ AH-1 tumor-specific CD8+ T cells, TNF-⍺+IFN-γ+ CD4+ and IFN-γ+IL2+ CD4+ effector T cells were higher than with the IL2c triple combination (n = 9) (p<0,05), particularly in the non-irradiated tumors.

CONCLUSION

RT+⍺PD1+⍺CTLA4 triple treatment induced more cytotoxic effector TILs than RT+⍺PD1+IL2c triple treatment, enhancing the abscopal effect and inducing a high abscopal cure rate. More experiments underway to reveal mechanisms.

Beyond checkpoint inhibition - Immunotherapeutical strategies in combination with radiation

The revival of cancer immunotherapy has taken place with the clinical success of immune checkpoint inhibition. However, the spectrum of immunotherapeutic approaches is much broader encompassing T cell engaging strategies, tumour-specific vaccination, antibodies or immunocytokines. This review focuses on the immunological effects of irradiation and the evidence available on combination strategies with immunotherapy. The available data suggest great potential of combined treatments, yet also poses questions about dose, fractionation, timing and most promising multimodal strategies.

Immunotherapy of patients with hormone-refractory prostate carcinoma pre-treated with interferon-gamma and vaccinated with autologous PSA-peptide loaded dendritic cells--a pilot study

PURPOSE

We conducted a pilot trial to assess the feasibility and tolerability of a prime/boost vaccine strategy using interferon-gamma (IFN-gamma) and autologous dendritic cells (DCs) pulsed with HLA-A2-specific prostate-specific antigen (PSA) peptides (PSA-1 [141-150]; PSA-2 [146-156]; PSA-3 [154-163]) for the treatment of 12 patients with hormone refractory prostate carcinoma.

PATIENTS AND METHODS

All patients were vaccinated four times with intracutaneously injected PSA-peptide loaded DCs after subcutaneous administration of IFN-gamma 2 hr before DC administration (50 microg/m(2) body surface). Objectives were safety, clinical benefit, clinical and biochemical response, quality of life, and immunological parameters.

RESULTS

The vaccination was well tolerated without any vaccination-associated adverse events. One partial and one mixed responder were identified, four patients showed stable diseases. Two patients had a decrease and four a slow-down velocity slope in the PSA serum level. All responders showed a positive DTH-response, but only two a slight increase in PSA-peptide specific T-lymphocytes.

CONCLUSION

The immunotherapy with IFN-gamma and PSA-peptide loaded DCs was feasible and well tolerated. The observed responses imply a potential antitumor activity.

Immunological functions of the proteasome

Proteasomes are highly abundant cytosolic and nuclear protease complexes that degrade most intracellular proteins in higher eukaryotes and appear to play a major role in the cytosolic steps of MHC class I antigen processing. This review summarizes the knowledge of the role of proteasomes in antigen processing and the impact of proteasomal proteolysis on T cell-mediated immunity.

Efficient MHC class I-independent amino-terminal trimming of epitope precursor peptides in the endoplasmic reticulum

MHC class I ligands are produced mainly by proteasomal proteolysis, in conjunction with an unknown extent of trimming by peptidases. Trimming of precursor peptides in the endoplasmic reticulum, a process postulated to be class I dependent, may substantially enhance the efficiency of antigen presentation. However, monitoring of luminal peptide processing has not so far been possible. Here we show that several precursor peptides with amino-terminal extensions are rapidly converted to HLA-A2 ligands by one or several highly efficient metallo-peptidases found on the outer surface of, but also within, microsomes. Surprisingly, luminal trimming is fully active in HLA class I- or TAP-deficient microsomes and precedes peptide association with HLA class I molecules. Trimmed peptides are rapidly depleted from, and become undetectable in, microsomes lacking the restricting class I molecules.

The specificity of proteasomes: impact on MHC class I processing and presentation of antigens

We have studied polypeptide processing by purified proteasomes, with regard to proteolytic specificity and cytotoxic T-lymphocyte (CTL) epitope generation. Owing to defined preferences with respect to cleavage sites and fragment length, proteasomes degrade polypeptide substrates into cohorts of overlapping oligopeptides. Many of the proteolytic fragments exhibit structural features in common with major histocompatibility complex (MHC) class I ligands including fragment size and frequencies of amino acids at fragment boundaries. Proteasomes frequently generate definitive MHC class I ligands and/or slightly longer peptides, while substantially larger peptides are rare. Individual CTL epitopes are produced in widely varying amounts, often consistent with immunohierarchies among CTL epitopes. We further found that polypeptide processing is remarkably conserved among proteasomes of eukaryotic origin and that invertebrate proteasomes can efficiently produce known high-copy MHC class I ligands, suggesting evolutionary adaptation of the transporter associated with antigen processing and MHC class I to ancient constraints imposed by proteasomal protein degradation.

A giant protease with potential to substitute for some functions of the proteasome

An alanyl-alanyl-phenylalanyl-7-amino-4-methylcoumarin-hydrolyzing protease particle copurifying with 26S proteasomes was isolated and identified as tripeptidyl peptidase II (TPPII), a cytosolic subtilisin-like peptidase of unknown function. The particle is larger than the 26S proteasome and has a rod-shaped, dynamic supramolecular structure. TPPII exhibits enhanced activity in proteasome inhibitor-adapted cells and degrades polypeptides by exo- as well as predominantly trypsin-like endoproteolytic cleavage. TPPII may thus participate in extralysosomal polypeptide degradation and may in part account for nonproteasomal epitope generation as postulated for certain major histocompatibility complex class I alleles. In addition, TPPII may be able to substitute for some metabolic functions of the proteasome.

Potential immunocompetence of proteolytic fragments produced by proteasomes before evolution of the vertebrate immune system

To generate peptides for presentation by major histocompatibility complex (MHC) class I molecules to T lymphocytes, the immune system of vertebrates has recruited the proteasomes, phylogenetically ancient multicatalytic high molecular weight endoproteases. We have previously shown that many of the proteolytic fragments generated by vertebrate proteasomes have structural features in common with peptides eluted from MHC class I molecules, suggesting that many MHC class I ligands are direct products of proteasomal proteolysis. Here, we report that the processing of polypeptides by proteasomes is conserved in evolution, not only among vertebrate species, but including invertebrate eukaryotes such as insects and yeast. Unexpectedly, we found that several high copy ligands of MHC class I molecules, in particular, self-ligands, are major products in digests of source polypeptides by invertebrate proteasomes. Moreover, many major dual cleavage peptides produced by invertebrate proteasomes have the length and the NH2 and COOH termini preferred by MHC class I. Thus, the ability of proteasomes to generate potentially immunocompetent peptides evolved well before the vertebrate immune system. We demonstrate with polypeptide substrates that interferon gamma induction in vivo or addition of recombinant proteasome activator 28alpha in vitro alters proteasomal proteolysis in such a way that the generation of peptides with the structural features of MHC class I ligands is optimized. However, these changes are quantitative and do not confer qualitatively novel characteristics to proteasomal proteolysis. The data suggest that proteasomes may have influenced the evolution of MHC class I molecules.

The proteolytic fragments generated by vertebrate proteasomes: structural relationships to major histocompatibility complex class I binding peptides

Proteasomes are involved in the proteolytic generation of major histocompatibility complex (MHC) class I epitopes but their exact role has not been elucidated. We used highly purified murine 20S proteasomes for digestion of synthetic 22-mer and 41/44-mer ovalbumin partial sequences encompassing either an immunodominant or a marginally immunogenic epitope. At various times, digests were analyzed by pool sequencing and by semiquantitative electrospray ionization mass spectrometry. Most dual cleavage fragments derived from 22-mer peptides were 7-10 amino acids long, with octa- and nonamers predominating. Digestion of 41/44-mer peptides initially revealed major cleavage sites spaced by two size ranges, 8 or 9 amino acids and 14 or 15 amino acids, followed by further degradation of the latter as well as of larger single cleavage fragments. The final size distribution was slightly broader than that of fragments derived from 22-mer peptides. The majority of peptide bonds were cleaved, albeit with vastly different efficiencies. This resulted in multiple overlapping proteolytic fragments including a limited number of abundant peptides. The immunodominant epitope was generated abundantly whereas only small amounts of the marginally immunogenic epitope were detected. The frequency distributions of amino acids flanking proteasomal cleavage sites are correlated to that reported for corresponding positions of MHC class I binding peptides. The results suggest that proteasomal degradation products may include fragments with structural properties similar to MHC class I binding peptides. Proteasomes may thus be involved in the final stages of proteolytic epitope generation, often without the need for downstream proteolytic events.

Human immune response to HIV-1 Nef. II. Induction of HIV-1/HIV-2 Nef cross-reactive cytotoxic T lymphocytes in peripheral blood lymphocytes of non-infected healthy individuals

HIV-specific CD8+ cytotoxic T lymphocytes (CTL) are thought to have a beneficial role in HIV infection. In a previous report we have shown that HIV-1 Nef-specific CTL can be readily induced in peripheral blood lymphocytes of seronegative healthy young adults by in vitro stimulation with autologous Epstein-Barr virus-transformed B lymphoblastoid cell lines transfected with the HIV-1 nef gene. Here we demonstrate that these Nef-specific CTL can efficiently lyse HIV-infected primary CD4+ T lymphocytes. CTL of the blood donor tested were Nef-specific and restricted by the autologous MHC class I molecules HLA-A2 and HLA-B7. They recognized HIV-1 Nef in association with both restriction elements but HIV-2 Nef only in association with HLA-B7. The cross-reactivity of the induced effector cells together with the potent immunogenicity of Nef in healthy seronegatives further support the inclusion of Nef as a constituent of HIV vaccines.

Contribution of proteasome-mediated proteolysis to the hierarchy of epitopes presented by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL) recognize peptide epitopes of protein antigens in a hierarchical fashion. We investigated whether proteolytic cleavage, in particular by proteasomes, is important in determining epitope hierarchy. Using highly purified 20S proteasomes, we find preferred cleavage sites directly adjacent to the N- and C-terminal ends of the immunodominant epitope of chicken ovalbumin, Ova257-264, while most of the subdominant epitope, Ova55-62, is destroyed by a major cleavage site located within this epitope. Moreover, we show that variations in amino acid sequences flanking these epitopes influence proteasomal cleavage patterns in parallel with the efficacy of their presentation. The results suggest that proteasomal cleavage within and adjacent to class I-restricted epitopes contributes to their level of presentation.

Human immune response to HIV-1-Nef. I. CD45RO- T lymphocytes of non-infected donors contain cytotoxic T lymphocyte precursors at high frequency

The immune response of peripheral blood lymphocytes (PBL) of non-exposed human individuals to the Nef protein of HIV-1 was studied. Nef is a regulatory protein of HIV which is immediately expressed after infection and which seems to be important in the pathogenicity of HIV. Nef may therefore serve as a potential target for effective immunity against HIV infection. Epstein-Barr (EBV)-transformed lymphoblastoid B cell lines (LCL) were established from four healthy young seronegative adults and transfected with the Nef gene. These cells served as stimulator cells for autologous PBL in vitro and as target cells for CTL. CTL responses were readily generated against Nef-transfected LCL, consisting of Nef-specific and putative EBV-specific CTL. Nef-specific CTL were generated exclusively from CD8+ cells and were MHC class I restricted. Since a vigorous Nef-specific CTL response in non-infected individuals was unexpected, CTL precursor frequencies were determined by limiting dilution analyses in non-fractionated PBL and in PBL separated into the CD45RO- (naive) and CD45RO+ (memory) T cell populations. As expected, the putative EBV-specific CTL precursors were predominantly found in the CD45RO+ subset at frequencies typical for memory T cells. Nef-specific CTL precursors, in contrast, were found predominantly in the CD45RO- population, at even higher frequencies of approximately 1/1000-1/3000. Nef may thus display either an unusually high number of immunogenic peptides or a limited number of peptides presented in a very efficient way, so that many T cells including low affinity cells, would be triggered.

Intracellular processing of hapten-modified protein for MHC class I presentation: cytoplasmic delivery by pH-sensitive liposomes

Phagocytic or endocytic uptake of pH-sensitive liposomes has been shown to result in the release of entrapped material into the cytosol. This system can therefore be applied to the targeted delivery of protein antigens into the MHC class I presentation pathway of antigen-processing cells. We have used trinitrophenyl (TNP)-modified chicken ovalbumin encapsulated in liposomes to examine the intracellular processing of haptenated proteins and the presentation of TNP-modified peptides to MHC class I-restricted hapten-specific CTL. Here, we demonstrate for the first time that hapten-modified proteins can undergo intracellular processing by macrophages, that similar peptides are produced in the form of unmodified or haptenated derivatives, and that TNP-peptides are transported to the cell surface and presented to class I-restricted CTL via the ER/Golgi pathway. This system can now be used to study T-cell responses to naturally processed hapten-conjugated peptides in vitro and in vivo.

Carboxyacyl derivatives of cardiolipin as four-tailed hydrophobic anchors for the covalent coupling of hydrophilic proteins to liposomes

Two carboxyacyl derivatives of cardiolipin, O-succinyl- and O-glutarylcardiolipin, were synthesized with the aim of using them as artificial membrane anchors for the immobilization of hydrophilic proteins to liposomes. Four adjacent fatty acid residues can be introduced into a protein with only one single amino group being blocked, by reacting the cardiolipin derivatives with the protein amino groups after carbodiimide activation. alpha-Chymotrypsin, used as a model protein, and modified with on average two molecules of O-succinylcardiolipin was incorporated into liposomes, which had been prepared by different methods, with very high yield. If incorporated in preformed liposomes, the carboxyacyl cardiolipin anchors were also efficient in binding proteins to liposomal surfaces. Up to 350 micrograms chymotrypsin/mumol lipid were coupled to small unilamellar vesicles, preserving reactivity of the enzyme towards specific macromolecular inhibitors. Human IgG could also be bound to anchor-containing liposomes with high protein to lipid coupling ratio as well as high coupling yield.

Thiolation of preformed liposomes with iminothiolane

Preformed phosphatidylethanolamine-containing liposomes were thiolated with 2-iminothiolane (Traut's reagent) and subsequently activated by mixed disulfide formation with 5,5'-dithiobis(2-nitrobenzoic acid). Up to 65% of amino groups of the outer liposomal lamella, corresponding to 230 SH-groups per vesicle, were modified. Covalent attachment of thiolated alpha-chymotrypsin to these thiol-liposomes via S-S bridges yielded a protein/lipid ratio of 3.6 X 10(-4) mol enzyme/mol lipid.