Does preventive vaccination with engineered tumor cells work in cancer-prone transgenic mice?

The use of genetically modified tumor cells as vaccines has been successful in numerous animal models of grafted syngenic tumors and has provided the groundwork for many clinical trials of gene therapy in cancer patients. To investigate the real efficacy of ex vivo gene therapy-based vaccines, we used transgenic mice that express the SV40 large T and small t antigens under the control of hepatic antithrombin III (ASV-B)-regulatory sequences. These mice systematically develop hepatocarcinoma. Hepatoma cells, derived from ASV-B transgenic mice, were gene-transduced to express either interleukin-2, interleukin-4, the granulocyte-macrophage colony-stimulating factor, or the T-cell costimulatory molecule B7.1. First, we demonstrated the vaccine potential of engineered hepatoma cells by immunizing nontransgenic mice with these cells, which prevented the growth of subsequent grafted nontransduced hepatoma cells. However, vaccination of pretumoral transgenic animals with various combinations of engineered hepatoma cells failed to inhibit hepatoma onset and progression. Rather, tumor development in ASV-B mice appears to be dependent on the immune system, since neonatal induction of immunotolerance to tumor in ASV-B mice cells was associated with a moderate, but significant, acceleration of tumor development. These results seriously call into question the efficacy of this strategy of active vaccinotherapy against natural tumors.

Type II and III receptors for immunoglobulin G (IgG) control the presentation of different T cell epitopes from single IgG-complexed antigens

T cell receptors on CD4(+) lymphocytes recognize antigen-derived peptides presented by major histocompatibility complex (MHC) class II molecules. A very limited set of peptides among those that may potentially bind MHC class II is actually presented to T lymphocytes. We here examine the role of two receptors mediating antigen internalization by antigen presenting cells, type IIb2 and type III receptors for IgG (FcgammaRIIb2 and FcgammaRIII, respectively), in the selection of peptides for presentation to T lymphocytes. B lymphoma cells expressing recombinant FcgammaRIIb2 or FcgammaRIII were used to assess the presentation of several epitopes from two different antigens. 4 out of the 11 epitopes tested were efficiently presented after antigen internalization through FcgammaRIIb2 and FcgammaRIII. In contrast, the 7 other epitopes were efficiently presented only when antigens were internalized through FcgammaRIII, but not through FcgammaRIIb2. The capacity to present these latter epitopes was transferred to a tail-less FcgammaRIIb2 by addition of the FcgammaRIII-associated gamma chain cytoplasmic tail. Mutation of a single leucine residue at position 35 of the gamma chain cytoplasmic tail resulted in the selective loss of presentation of these epitopes. Therefore, the nature of the receptor that mediates internalization determines the selection of epitopes presented to T lymphocytes within single protein antigens.

Accumulation of the p53 protein allows recognition by human CTL of a wild-type p53 epitope presented by breast carcinomas and melanomas

The p53 protein is accumulated in tumor cells of many human cancers and can elicit in vivo humoral and proliferative responses. Rare reports about p53-mediated tumor recognition by CTLs have remained questioned. We therefore studied a panel of breast tumor and melanoma cell lines that we assayed for the presence of accumulated p53 and surface HLA-A2 and for the presentation of p53 epitopes. From PBMC of a healthy donor, we have generated a CTL line, D5/L9V, directed against HLA-A2-restricted peptide 264-272 from wild-type p53. It efficiently lysed breast adenocarcinomas MCF-7, MCF7/RA1, and MDA-MB-231, and melanoma M8, which all accumulate the p53 protein. Using competition assays, we made sure that tumor lysis by D5/L9V was due to recognition of endogenously produced p53 peptide 264-272 associated with the HLA-A2.1 molecule on the surface of these tumor cells. Cells with undetectable levels of wild-type p53, such as lymphoblastoid cells and melanoma M74, were not recognized by D5/L9V. Neither were breast tumor cell line MCF7/ADR nor melanoma line M44 because of HLA loss. This study therefore shows that it is possible to obtain in vitro CTL lines that specifically recognize a p53 epitope spontaneously presented by a variety of HLA-A2+ transformed cell lines provided they display abnormal patterns of p53 expression. This work points out that breast tumors and melanomas share a p53 epitope, and raises hopes for future immunotherapeutic approaches.

Critical stages of tumor growth regulation in transgenic mice harboring a hepatocellular carcinoma revealed by distinct patterns of tumor necrosis factor-alpha and transforming growth factor-beta mRNA production

There is now good evidence that cytokines contribute to the regulation of tumor growth. The cytokine-driven modulation of tumor growth was investigated during the progression of a hepatocellular carcinoma (HCC) in SV40 large T tumor antigen transgenic mice. In vivo, an increased rate of liver growth correlated with increased transforming growth factor (TGF)-beta 1 mRNA expression, while the greatest amounts of tumor necrosis factor (TNF)-alpha mRNA were detected earlier during tumor development. Conversely, no particular alteration of IL-1 alpha, IL-1 beta, IL-6, IL-2, IL-4 and IFN-gamma mRNA production could be reported. In vitro, hepatocyte-like tumor cell lines established at two stages, either before or after HCC differentiation, were characterized. The early-stage-derived cell line produced TNF-alpha mRNA, but had barely detectable expression of TGF-beta 1 mRNA, while later-stage-derived cell lines showed the reciprocal pattern. All cell lines displayed a lack of sensitivity to TNF-alpha, although some degree of sensitivity to TNF-alpha could be observed in the presence of actinomycin-D or after treatment with IFN-gamma. The early-stage-derived cell line was sensitive to the growth inhibitory effects of TGF-beta 1, but late-stage-derived tumor cell lines displayed a loss of sensitivity to TGF-beta 1 which correlated with the increased expression of TGF-beta 1 mRNA. Altogether, this suggests that tumor cells contribute to the discrete TNF-alpha and TGF-beta 1 expression patterns during HCC progression. This model of HCC could be of valuable interest to assess the impact of various immunotherapeutic strategies on modulation of tumor growth.

In vivo T helper cell response to retro-inverso peptidomimetics

Peptide analogues containing reversed peptide bonds between each residue along the peptide sequence (retro-inverso modification) have been analyzed for their antigenic and in vivo immunogenic properties in the MHC II and Th cell response context. Two antigenic peptides were selected for this study, namely peptide 103-115 of poliovirus VP1, which is involved in the production of Abs that neutralize the infectivity of the virus, and peptide 435-446 from the third constant region of mouse heavy chain IgG2a allopeptide gamma 2ab, which mimics a corneal Ag implicated in autoimmune keratitis. In a competition assay performed in vitro using reference hybridomas of known MHC class II restriction, both retro-inverso analogues bound (although more weakly in our test) to I-Ad and/or I-Ed class II molecules. However, in both cases, this lower affinity was apparently largely compensated in vivo, as a T cell response (with IL-2 secretion), equivalent to that obtained with the wild-type peptides, was observed following immunization of BALB/c mice with the retro-inverso analogues. Moreover, these T cells proliferated and produced IL-2 in response to the cognate peptides. It is concluded that the T cell receptors of T cells primed in vivo with the retro-inverso analogues readily cross-react with parent and retro-inverso analogue-MHC complexes. The approach of using pseudopeptides containing changes involving the backbone, and not the orientation of side chains, may thus be promising to design potent immunogens for class II-restricted T cells.

In vivo T helper cell response to retro-inverso peptidomimetics

Peptide analogues containing reversed peptide bonds between each residue along the peptide sequence (retro-inverso modification) have been analyzed for their antigenic and in vivo immunogenic properties in the MHC II and Th cell response context. Two antigenic peptides were selected for this study, namely peptide 103-115 of poliovirus VP1, which is involved in the production of Abs that neutralize the infectivity of the virus, and peptide 435-446 from the third constant region of mouse heavy chain IgG2a allopeptide gamma 2ab, which mimics a corneal Ag implicated in autoimmune keratitis. In a competition assay performed in vitro using reference hybridomas of known MHC class II restriction, both retro-inverso analogues bound (although more weakly in our test) to I-Ad and/or I-Ed class II molecules. However, in both cases, this lower affinity was apparently largely compensated in vivo, as a T cell response (with IL-2 secretion), equivalent to that obtained with the wild-type peptides, was observed following immunization of BALB/c mice with the retro-inverso analogues. Moreover, these T cells proliferated and produced IL-2 in response to the cognate peptides. It is concluded that the T cell receptors of T cells primed in vivo with the retro-inverso analogues readily cross-react with parent and retro-inverso analogue-MHC complexes. The approach of using pseudopeptides containing changes involving the backbone, and not the orientation of side chains, may thus be promising to design potent immunogens for class II-restricted T cells.

Homogeneous immunoassays using rare earth cryptates and time resolved fluorescence: principles and specific advantages for tumor markers

We have developed a new type of long lived fluorescent tracer, the rare earth cryptates formed by the inclusion of a Eu3+ ion into the intra molecular cavity of a macrobicyclic ligand containing bipyridine units. This tracer is used in a new homogeneous assay format based on spectral and temporal selectivity as well as on an amplification of the cryptate fluorescence. As a consequence of these features, the measurement of the specific signal is totally shielded from media interaction and corrected in real time for sample to sample optical variation. We show the example of a PSA assay where an analytical sensitivity of 0.03 ng/ml is obtained and an AFP assay where measurements in the first minutes of the incubation allow a rapid estimation of the AFP concentration and therefore an immediate dilution of the samples if required. These results illustrate the performance of this new homogeneous method and point out the specific advantages it allows in terms of sensitivity, precision and flexibility. It is therefore particularly well adapted for the assay of analytes like tumor markers where both sensitivity and wide dynamic range are needed.

A wild-type p53 cytotoxic T cell epitope is presented by mouse hepatocarcinoma cells

The possibility to identify epitopes presented by tumor cells to cytotoxic T lymphocytes (CTL) has given rise to new fields in tumor immunology. The tumor suppressor gene product p53 is a good candidate antigen because it is involved in the tumorigenesis of many cancers. It accumulates in an inactivated form due to mutation or formation of heterodimers with an oncogene product. Epitopes from the mutant or wild-type p53 proteins are thought to be presented by tumor cells and to induce a tumor-specific CTL response. To identify such epitopes, mouse wild-type p53 peptides encompassing the H-2 Db anchoring motif were tested for their association with the Db molecule. Positive peptides were assayed for their ability to induce CTL in C57BL/6 mice. CTL specific for one wild-type p53 peptide, p232-240, were isolated and found to lyse hepatocarcinoma cell lines established from mice transgenic for simian virus 40 large T antigen which overexpress p53. These results show that the p232-240 epitope from wild-type p53 is naturally processed and presented in H-2b tumor cells.

Epitope analysis of T- and B-cell response against the human beta 1-adrenoceptor

Several reports have recently raised the possible significance of the presence of autoantibodies against the beta 1-adrenoceptor in patients with idiopathic dilated cardiomyopathy. An investigation was thus initiated to study the immune response against this receptor at the T-cell and the B-cell level. Using membranes of E coli transfected with the human beta 1-adrenoceptor gene as immunogen, T-helper cells of the immunized mice were stimulated with synthetic peptides derived from the receptor and predicted to be immunogenic to assess the T-cell immunodominant regions of the receptor. Three peptides derived from the second transmembrane region, from the second extracellular loop and from the C-terminal domain were shown to be stimulatory. Synthetic peptides, derived from two domains of the receptor which could be potential targets for autoantibodies, yielded an antibody response after immunization with the free peptides. The peptide derived from the N-terminal region yielded antibodies which recognized the receptor in immunoblot and by immunoprecipitation but they had no functional effect on the receptor. The peptide derived from the second extracellular loop yielded antibodies which recognized the receptor in immunoblot and by immunoprecipitation of the free receptor and which had a pharmacological effect on the receptor. The second extracellular loop thus contains T- and B-cell epitopes which could be involved in the autoimmune process.

Induction of mouse syngeneic MLR by in vivo xenogeneic immunization with HLA-DR antigens

To study in mice the effects of in vivo xenogenic immunization with human major histocompatibility complex (MHC) class II antigens, the animals were injected with HLA-DR antigens and their proliferative responses tested in vitro. The results showed that small amounts of HLA-DR proteins, acting as nominal antigens, were not only able to prime mice for a secondary in vitro xenogenic mixed lymphocyte reaction but also induced a syngeneic mixed lymphocyte reaction. In contrast, allogeneic or syngeneic immunization of mice with soluble MHC class II molecules failed to stimulate an autoreactive response. The syngeneic mixed lymphocyte reaction was primarily directed against syngeneic MHC class II molecules since the murine T lymphocytes reacted against MHC class II-positive dendritic spleen cells and MHC class II-transfected mouse fibroblasts. A self-reactive T-cell line induced under these experimental conditions did not react in xenogeneic and allogeneic mixed lymphocyte reactions. However, these T lymphocytes proliferated when human peripheral blood lymphocytes of various haplotypes were presented in the context of syngeneic mouse antigen presenting cells.

Xenogeneic recognition of soluble and cell surface HLA class II antigens by proliferative murine T cells

In order to characterize the murine anti-human xenogeneic mixed lymphocyte reactions (MLR), we studied T cell proliferative responses against various human lymphoid cells by immunization of mice either with cellular or purified HLA-DR antigens. Data presented here indicated that small amounts of soluble HLA-DR antigen were able to prime mice, and that the xenogeneic MLR depends on the expression of HLA class II antigens on the stimulating cells. Experiments using a mutant cell line clearly showed that HLA-DP molecules were also sufficient in eliciting a primary or a secondary xenogeneic MLR while no secondary proliferative response was obtained with cells expressing only HLA class I molecules. Using a large panel of human cells with various haplotypes, our results also showed that (a) nonpolymorphic determinants of HLA class II antigens trigger dominantly the murine T cells and (b) the xenogeneic response required I-E and L3T4 accessory molecules and was not inhibited with anti I-A and monomorphic anti-HLA class II antigen monoclonal antibodies. Altogether these results suggest that HLA class II antigens act as nominal antigens in triggering a murine anti-human proliferative response.

Monoclonal B-cell response to diphtheria toxoid: evidence for cross-reactive epitopes

Our previous studies on the human specific B- and T-cell responses against diphtheria toxoid (DT) had shown that the B- and T-cell antigenic determinants were mostly assembled topographic sites with large intra-molecular cross-reactivities. In order to characterize further the B-cell responses against DT, and to investigate these cross-reactivities, we have produced 54 murine monoclonal antibodies (moAbs) against the native DT. Determination of their antigenic specificity showed that 25 moAbs recognized conformational determinants on the native DT, 20 moAbs reacted against the A fragment (DTA) of the toxin, and nine moAbs reacted with at least three distinct highly purified CNBr peptides of the B fragment (DTB); some of them simultaneously recognized two or three purified CNBr peptides of DTB. As shown by cross-inhibition studies, the epitopes cross-reacting with those moAbs were distinct. Previously described sequence data indicated that these epitopes were not induced by repetitive amino-acid sequences. Finally, a cross-reactive idiotype was shared by moAbs specific for the CB1 and the CB3 peptides of DTB. Altogether, these data indicate that anti-DT moAbs are mostly directed against conformational determinants, and may cross-react with several DTB CNBr peptides.