Protection of mice against cancer by immunization with membranes but not purified virions from virus infected cancer cells

The genetics of alphaviruses

Alphaviruses are emerging human pathogens that are transmitted by arthropod vectors. Their ability to infect a wide range of vertebrate hosts including humans, equines, birds and rodents has brought about a series of epidemic and epizootic outbreaks worldwide. Their potential to cause a pandemic has spurred the interest of researchers globally, leading to the rapid advancement on the characterization of genetic determinants of alphaviruses. In this review, the focal point is placed on the genetics of alphaviruses, whereby the genetic composition, clinical features, evolution and adaptation of alphaviruses, modulation of IFN response by alphavirus proteins and therapeutic aspects of alphaviruses will be discussed.

Alphavirus vectors for cancer therapy

Alphaviruses contain a single strand RNA genome that can be easily modified to express heterologous genes at very high levels in a broad variety of cells, including tumor cells. Alphavirus vectors can be used as viral particles containing a packaged vector RNA, or directly as nucleic acids in the form of RNA or DNA. In the latter case alphavirus RNA is cloned within a DNA vector downstream of a eukaryotic promoter. Expression mediated by these vectors is generally transient due to the induction of apoptosis. The high expression levels, induction of apoptosis, and activation of type I IFN response are the key features that have made alphavirus vectors very attractive for cancer treatment and vaccination. Alphavirus vectors have been successfully used as vaccines to induce protective and therapeutic immune responses against many tumor-associated antigens in animal models of mastocytoma, melanoma, mammary, prostate, and virally induced tumors. Alphavirus vectors have also shown a high antitumoral efficacy by expressing antitumoral molecules in tumor cells, which include cytokines, antiangiogenic factors or toxic proteins. In these studies induction of apoptosis in tumor cells contributed to the antitumoral efficacy by the release of tumor antigens that can be uptaken by antigen presenting cells, enhancing immune responses against tumors. The potential use of alphaviruses as oncolytic agents has also been evaluated for avirulent strains of Semliki Forest virus and Sindbis virus. The fact that this latter virus has a natural tropism for tumor cells has led to many studies in which this vector was able to reach metastatic tumors when administered systemically. Other "artificial" strategies to increase the tropism of alphavirus for tumors have also been evaluated and will be discussed.

Prevention of metastatic spread by postoperative immunotherapy with virally modified autologous tumor cells. I. Parameters for optimal therapeutic effects

Effective anti-metastatic therapy was achieved in a mouse tumor model by combining surgery with post-operative immunotherapy using virus-modified autologous tumor cells. No therapeutic effect was observed when using for immunotherapy the nonmodified autologous tumor ESb, which is only weakly immunogenic and highly metastatic. The viral modification was achieved by infecting the tumor with an avirulent strain of Newcastle disease virus (NDV), which led to expression of viral antigens and to an increase in the tumor cells' immunogenicity. Parameters which were of decisive influence for success or failure of therapy were the time of operation of the primary tumor and the dose of virus which was admixed to a standard dose of irradiated tumor cells. There was a low dose optimum of NDV at about 100 hemagglutinating units per 25 X 10(6) tumor cells. The therapeutic effect observed was less pronounced if the virus was given separately from the tumor cells. Post-operative immunotherapy with NDV-modified tumor cells had the following therapeutic effects: (1) disappearance of micrometastases from visceral organs as ascertained by a sensitive bioassay; (2) life prolongation in virtually all animals when compared to controls (operated only); (3) cures in about 50% of the treated animals. The possible mechanism of this therapeutic effect and its potential for clinical application are discussed.

Augmented immunogenicity of Lewis lung carcinoma by infection with herpes simplex virus type 2

In vitro, LLT cells sustain HSV-2 replication without evidence of lysis. Simultaneously, multiplication of the cells is stimulated. These xenogenized cells were tested for their immunopotentiating capacity: three-step immunization with xenogenized viable cells conferred significantly augmented transplantation resistance to a challenge graft with 4 x 10(4) intact LLT cells. Latency periods preceding tumor formation were increased and 15% of the mice failed to form primary tumors. Metastasis was likewise decreased and 25% of the mice had healthy lungs. Immunopotentiation, however, did not suffice to significantly protect against a challenge with 6 x 10(4) intact cells. The presence of virus-specific neoantigens on HSV-2-infected viable cells was demonstrated by the progressively increasing number of rejections of 4 x 10(4) xenogenized cells during the successive immunization steps. Immunization with non-viable LLT cells did not augment resistance to challenge.

Melanoma skin test antigens of improved sensitivity prepared from vesicular stomatitis virus-infected tumor cells

Crude membrane (CM) extracts from three different cultured human melanoma lines that were "virus-augmented" (infected with vesicular stomatitis virus (VSV) and subsequently inactivated by ultraviolet light) produced positive skin tests in 17 of 20 (85%), 11 of 20 (55%), and 13 of 18 (72%) tests, respectively, performed in 20 melanoma patients. Identical CM extracts from the same melanoma lines that had not been infected with VSV gave positive skin tests in 2 of 20 (10%), 4 of 20 (20%), and 2 of 18 (11%) tests, respectively, performed in the 20 melanoma patients, and no positive tests in the control patients. The 3 virus-augmented extracts were positive in only 2 of 18 (11%), 0 of 18 (0%), and 1 of 17 (6%) control subjects, respectively. The controls consisted of six normal volunteers and 12 patients with cancers other than melanoma. The "virus-augmented" CM extracts thus exhibited markedly greater sensitivity without significant loss of specificity as compared to nonvirus augmented extracts when used as tumor-specific melaonma skin test antigens.

Growth, purification and characterization of Semliki Forest virus in Ehrlich ascites tumor cell suspensions

The growth of Semliki Forest Virus (SFV) in suspension cultures of Ehrlich Ascites (EA) cells and its purification is described. Large volumes of virus material were concentrated by filtration with DIAFLO XM-300 membrane and precipitation with ammonium sulfate. A combination of protamine sulfate treatment, centrifugation of the virus onto a 50 per cent sucrose cushion, and sedimentation through a 5--30 per cent sucrose density gradient was employed. The purified virus particles were homogeneous as revealed by electron microscopy, by moving boundary electrophoresis, and by polyacrylamide gel electrophoresis. Virus suspensions containing 1 mg/ml of protein had a hemagglutinin titer of 1:12,000 when measured with 0.25 per cent goose red blood cells.

Augmented immunogenicity of tumor cell membranes produced by surface budding viruses: parameters of optimal immunization

Membranes prepared from tumor cells infected with surface budding viruses are much more immunogenic than membranes from uninfected tumor cells. Factors affecting immunization with membranes from virus-infected tumor cells were studied. Preparations made with influenza virus were clearly superior to those prepared with vesicular stomatitis virus (VSV). Membranes infected with VSV were maximally immunogenic at a dose equivalent to a 10% cell pack whereas influenza-virus-infected membranes were immunogenic at 1/100th of this dose. Subcutaneous inoculation was better than other routes of administration. Maximum protection against challenge with viable tumor cells was afforded by two inoculations of VSV-infected membranes spaced 3 days apart or a single inoculation with influenza-virus-infected membranes. Administration of membranes in complete Freund's adjuvant either had no effect of induced a slight degree of tumor enhancement. Immunization with influenza-virus-infected membranes significantly reduced tumor size and incidence even at a challenge dose of tumor cells which was 50 times the LD100.