Non-immunogenicity of enucleated rat hepatoma cells in syngeneic animals

In vitro cellular immune response to measles viral glycoproteins: role of the antigen vector

The capacity of measles virus and haemagglutinin (HA) and fusion (F) glycoproteins, presented either as soluble antigens, associated with liposomes or as Iscoms, to induce an in vitro primary and anamnestic cellular response was studied in syngeneic W/Fu rats using the MTT colorimetric assay. A primary cellular response was observed when the virus, HA + F liposomes or HA + F Iscoms were used as immunogens, but not when soluble HA + F glycoproteins were used. The irradiation of the naïve spleen cells at 500 rads allowed the generation of a primary response with the soluble antigens. All these primary responses were low, of similar intensity and dose-dependent. The responses were stronger after the stimulation of spleen cells from seropositive immune rats with virus, HA + F liposomes or HA + F Iscoms, whereas they were moderate after stimulation with soluble HA + F. In addition, far less antigenic material was required and the use of a vehicle for HA + F (liposomes, Iscoms) dramatically lowered the threshold of the sensitivity to the antigens. The immunization of rats with soluble HA + F glycoproteins resulted in the anergy of their spleen cells even to virus, HA + F liposomes or HA + F Iscoms. Again, irradiation of these cells could restore their ability to elicit a primary response to any type of HA + F immunogens. Using the lysosomotropic Leu-0-Met agent, all the cellular responses were found to be accessory cells dependent, the responses being restored after supplementation with 10% peritoneal exudate cells from naïve rats. These treatments did not break the immunosuppression induced by soluble HA + F glycoproteins. The uptake of the various immunogens by the murine macrophage cell line J774-1 was also studied using radioactively labelled virus and HA + F glycoproteins. The uptake of soluble HA + F was limited to 10-15%, whereas that of the other immunogens was almost complete. The data reported indicate that the modification of the supramolecular architecture of HA + F glycoproteins by their presentation in liposomes or Iscoms could modulate their immunogenicity, both qualitatively and quantitatively. It could prevent the generation of a radiosensitive suppressive mechanism, increase the sensitivity to the antigen and the activation level of the responding cell population. Quantitative modifications are accessory cell dependent and initiated within the first hour of the stimulation.

Enhancement of immunogenicity of tumour virus antigen by liposomes: the effect of lipid composition

The secondary humoral response evoked in W/Fu rats by the weakly immunogenic soluble Gross cell surface antigen (GCSAa) extracted from the syngeneic (C58NT)D lymphoma can be enhanced when GCSAa is presented in liposomes, and this requires the antigen to be strongly associated with the phospholipid bilayers. In order to investigate further the role of the phospholipid microenvironment in the membrane presentation of this antigen, the relationship between the phospholipid composition and the immunogenic potency of GCSAa liposomes was explored. For a given neutral phospholipid component, optimal immunogenicity was obtained when 20% cholesterol was present and when a negatively charged phospholipid was included as a minor component. When phosphatidylcholines (PC) were used as the major neutral component, the immunogenicity of GCSAa liposomes from optimal for distearoyl PC (DSPC) decreased when decreasing the PC acyl chain length down to the background level for dilauroyl PC (DLPC). Similarly, the use of PC with acyl chain of increasing unsaturation was followed by a decline in the immunogenicity of the GCSAa liposomes up to the background level for dilinoleoyl PC (DLiPC). Replacing PC headgroups by phosphatidylethanolamine (PE) headgroups abolished the enhancing effect of the liposome presentation on GCSAa immunogenicity. Three groups of phospholipids unable to promote the expression of the GCSAa immunogenicity could be distinguished: the DLPC, DLiPC group, unable to prime the animals but, contrary to the soluble antigen, able to boost the animal after an appropriate priming with GCSAa-DSPC-liposomes, the dimyristoyl PE group, unable to prime or to boost and non-toxic in vitro for the macrophages, and the dipalmitoyl PE group, acutely toxic for macrophages.

Presentation of an MuLV-related tumour antigen in liposomes as a potent tertiary immunogen after adoptive transfer

The immunogenicity of Gross virus cell surface antigen (GCSAa) extracted from rat lymphoma cells can be dramatically increased by its presentation into liposomes, probably by mimicking the cell membrane presentation. Induction of an anti-GCSAa secondary antibody response has been found to require the use of liposomes as GCSAa vehicle for both the priming and the boosting immunizations. In order to investigate the sensitivity of highly immune cells to the liposome presentation, immune spleen cells were stimulated in vitro with either soluble GCSAa or GCSAa-liposomes and transferred together with the immunogen into syngeneic animals. Only spleen cells from high responders, which have been immunized twice with GCSAa-liposomes, were able to generate an antibody response in naive recipients after their restimulation with the GCSAa-liposome preparation. Their restimulation with soluble antigen was ineffective unless 10% peritoneal exudate cells (PEC) from naive rats were added during the in vitro incubation. Stimulation with GCSAa-liposomes was further improved by the addition of 10% PEC. Macrophages were found to play a central role in the induction of antibody response in the recipients after stimulation with GCSAa-liposomes. Treatment of immune spleen cells with the macrophage-killing agent leucine methyl ester prior their restimulation in vitro with GCSA-liposomes in the absence of PEC, or depletion of macrophage after their in vitro incubation with this immunogen, completely abolished the induction of anti-GCSAa antibodies in the recipients.

Modification of cell cholesterol content of rat tumour cells. Effect upon their tumorigenicity and immunogenicity

Among the different constituents of the cell membrane, lipids have been poorly studied with respect to their role in the immunogenicity of tumour cells and their influence on the expression on tumour-associated antigens. Since liposome-associated antigens are more potent immunogens when the lipid matrix is in a rigid state, we have modified the lipid composition of rat hepatoma cells by incorporation of cholesteryl hemisuccinate (CH) into the lipid matrix, and studied its effect on the tumorigenicity and immunogenicity of these tumour cells in syngeneic animals. A slight and significant decrease of tumorigenicity of CH-enriched D23 cells was observed when 2×103 cells were injected SC, whereas with a higher tumour cell challenge there was no difference in the tumorigenicity of untreated or treated cells. The immunogenicity of CH-treated cells was tested by IP immunization with 107 or 106 cells followed 1 week later by an SC challenge with 2×104 viable D23 cells. No statistical difference was observed between the immunogenicity of CH-enriched cells and that of control cells on either tumour incidence or tumour growth rate. In addition, similar experiments performed with the spontaneous mammary carcinoma SP4 showed that CH-enriched SP4 cells were of lower immunogenicity and unable to induce a significant memory immunity. This lack of effect of the CH treatment on the immunogenicity was not related to the absence of incorporation of CH, since the CH treatment increased the cell lipid rigidity as determined by the increase of fluorescence anisotropy of the diphenyl hexatriene probe. These results obtained in two weak immunogenic tumour models underlined the need for further studies before such a lipid modification of cancer cells is applied in human immunotherapy trials.