A Shope Fibroma virus PYRIN-only protein modulates the host immune response.

PYRIN domain (PYD) proteins have recently emerged as important signaling molecules involved in the development of innate immunity to intracellular pathogens through activation of inflammatory mediator pathways. ASC is the central adaptor protein, which links pathogen recognition by PYD-containing pathogen recognition receptors to the activation of downstream effectors, including activation of Caspase-1 and NF-kappaB. The cellular PYD-only protein 1 (cPOP1) can block the recruitment of ASC to activated PAN receptors and thereby functions as an endogenous inhibitor of the PYD-mediated signal transduction pathway. Here we describe the identification and characterization of a Shope Fibroma homolog to cPOP1. Like cPOP1, a Shope Fibroma virus-encoded POP (vPOP), co-localizes and directly associates with ASC and inhibits PYD-mediated signal transduction. Poxviruses are known to encode immune evasive proteins to promote host cell infection and suppression of the host immune response. Poxvirus-encoded vPOPs represent a novel class of immune evasive proteins and impair the host response by blocking Cryopyrin and ASC inflammasome-mediated activation of pro-Caspase-1 and subsequent processing of pro-interleukin (IL)-1beta, and expression of vPOPs causes activation of NF-kappaB.

Virus-induced loss of class I MHC antigens from the surface of cells infected with myxoma virus and malignant rabbit fibroma virus.

Shope fibroma virus (SFV) is a leporipoxvirus that causes localized benign fibromas in immunocompetent adult rabbits that spontaneously regress due, in part, to a cell-mediated immune response. Myxoma virus (MYX) and malignant rabbit fibroma virus (MRV) are related leporipoxviruses that induce rapidly lethal generalized infections accompanied by tumors and immunosuppression. Because only these latter two viruses are known to compromise cell-mediated antiviral responses, cell surface levels of class I MHC molecules in SFV-, MRV-, and MYX-infected cells were investigated by fluorescent activated cell sorting analysis using a variety of different anti-HLA mAb. After infection with MYX or MRV there is a rapid decrease in the levels of detectable surface class I epitopes as detected by each antibody and by 24 h postinfection class I MHC Ag levels at the cell surface approach the level of background fluorescence observed with control antibodies. In contrast, only a moderate class I decrease is seen during infection with either SFV or vaccinia virus, an orthopoxvirus that is neither tumorigenic nor immunosuppressive. Surface class I marker loss induced by MYX and MRV is not simply due to nonspecific inhibition of total cellular protein synthesis by the viruses because class I levels decrease much further than the extent measured by estimating surface marker turnover in the presence of the protein synthesis inhibitor cycloheximide. Thus the loss of cellular surface class I molecules greatly exceeds the drop in level caused by complete blockage of host cell gene expression, and must involve removal or masking of preexisting class I epitopes from the cell surface by MRV/MYX. Cell surface levels of the transferrin receptor are unaffected by MYX and MRV infection, suggesting the observed class I decrease is not a nonspecific effect on total cell surface glycoproteins. Analysis of cells infected with MRV/MYX in the presence of cycloheximide or of cytosine arabinoside, an inhibitor of poxviral DNA replication, indicates that the class I marker loss is mediated in part by one or more viral late gene products. A probable explanation is that MRV/MYX late protein(s) interact with the class I MHC complex to either physically sequester these away from the cell surface and inhibit their recycling or else induce a conformational change that precludes recognition by all class I antibodies tested. In either event, we propose that such a major perturbation of the class I MHC complex would likely downregulate the class I-mediated presentation of viral Ag required to initiate cell-mediated immunity to these viruses.

Sequence and analysis of a portion of the genomes of Shope fibroma virus and malignant rabbit fibroma virus that is important for viral replication in lymphocytes.

The 10.7-kb BamHI "C" restriction fragment of malignant rabbit fibroma virus (MV) contains genes that are important for its immunosuppressive activity. When this fragment is transferred to a related avirulent leporipoxvirus, Shope fibroma virus (SFV), recombinant viruses show clinical features characteristic of MV: they replicate in lymphocytes and alter immune function in vitro, induce disseminated tumors in recipient rabbits, and are immunosuppressive in vivo. The 10.7-kb BamHI "C" restriction fragment of MV was sequenced in its entirety. Its DNA sequence and the 14 ORF's derived from analyzing this sequence are discussed. Analysis of known open reading frames to which the ORF's from MV's Bam "C" fragment show homology permits us to identify some MV ORF's showing high degrees of similarity to known and postulated proteins produced by vaccinia virus. Functions for some of these vaccinia proteins are known, while functions for others are hypothetical or unknown. Further analysis of genetic determinants of MV's virulence has indicated that two overlapping restriction subfragments of the BamHI "C" fragment can transfer MV's virulent behavior to SFV. The 0.7-kb region in which these two subfragments overlap includes the C-terminus of MV orf C-7 and the N terminus of MV orf C-8. These correspond to the C- and N-termini, respectively, of SFV orf's D-9 and D-10 and to vaccinia orf's D-6 (early transcription factor) and D-7 (subunit of RNA polymerase). We sequenced the region of SFV's BamHI "D" fragment in this area and illustrate here the comparative sequences of this portion of SFV's genome and orf's. On the basis of comparisons between MV, SFV, and vaccinia in this area we discuss the potential significance of these observations.

Syphilis superinfection activates expression of human immunodeficiency virus I in latently infected rabbits.

Superinfection of latently human immunodeficiency virus (HIV)-infected rabbits with either Treponema pallidum or Shope fibroma virus (SFV) activates HIV expression. In addition, HIV-infected rabbits demonstrate prolonged cutaneous lesions (chancres) after intracutaneous challenge with T. pallidum, the causative agent of syphilis. Rabbits were infected by intravenous inoculation of 3 x 10(7) human T-cell lymphotrophic virus type III (HTLV-III)/B10 (HIV-1)-infected H9 (human) cells. Five weeks after initial infection, integrated HIV-1-specific DNA sequences were detected in the DNA of the peripheral blood lymphocytes of only one of eight rabbits using polymerase chain reactions (PCR); human DNA could not be detected at this time. Furthermore HIV infection could not be demonstrated by either seroconversion or PCR during the next 6 months. All HIV-infected rabbits remained clinically healthy and had normal white blood cell counts. Six months after HIV infection, four HIV-infected and two noninfected controls were superinfected with 10(6) T. pallidum in eight skin sites in the shaved skin of the back, and four infected and two control animals were challenged with an intradermal injection with SFV. After infection with either syphilis or SFV, the DNA from the white blood cells of all eight HIV-infected rabbits contained HIV sequences, and HIV sequences were demonstrated in dermal mononuclear cells of the syphilitic lesions by in situ hybridization. The SFV-induced tumors were rejected normally in the HIV-infected rabbits, but four of the four rabbits challenged with T. pallidum had delayed development of cutaneous lesions and three of four demonstrated larger and more prolonged lesions. White blood counts, mitogen responses, and interleukin-2 production remained within normal limits, and seroconversion for HIV was not detected. Three of four rabbits in a second group, challenged with T. pallidum 4 months after HIV-inoculation, also had delayed healing of syphilitic lesions. These results indicate that latent HIV-infection of rabbits may be activated by immunostimulation and that latently HIV-infected rabbits have impaired delayed hypersensitivity reactions. It is hypothesized that true latent HIV-infection in the rabbits is in monocytes and postulated that further immunostimulation may produce infection of lymphocytes and activation of disease.

Determinants of the ability of malignant fibroma virus to induce immune dysfunction and tumor dissemination in vivo.

The relationship of virus-induced immunological dysfunction and tumor dissemination was studied using two related tumor-causing leporipoxviruses: malignant fibroma virus (MV) and Shope fibroma virus (SFV). Recombinant viruses, produced by transferring MV's 10.7 kb BamHI C fragment to SFV, replicate in lymphocytes and suppress lymphocyte function in vitro. Those recombinants that replicate in lymphocytes and suppress lymphocyte function in vitro share about 3.5 kb from MV's C fragment. Some recombinants mimic MV in producing immune suppression and disseminated virus infection in vivo. Other recombinants, even some that are highly immunosuppressive in vitro (e.g. R71), only variably induce immune suppression in vivo, and do not cause disseminated disease. A segment of DNA from MV that transfers to Shope fibroma virus almost all of MV's virulence in vivo was identified.

Trichinella spiralis as a modulator of Shope fibroma virus.

After the works on the promoting effect of trichinellosis on some viral infections in rodents, many studies successively demonstrated that Trichinella spiralis confers resistance to many unrelated antigens including pathogens, such as Protozoa, Bacteria and tumour cells (B16 melanoma). Considering the above contradictory results, the present work was undertaken to study, in rabbits, T. spiralis as a modulator of Shope's fibroma virus, an oncogenic virus responsible for a benign neoplasia. Four groups of 6 rabbits each were used. The rabbits of group I, II and III were inoculated per os with 3000; 6000 and 12,000 T. spiralis larvae, respectively. The rabbits of group IV were used as controls. Thirty-five days after the inoculation, all the animals were injected at the fixed doses of 0.5 ml with dilutions (10(-1) to 10(-8] of Shope's fibroma virus given intradermally into 8 different points of the skin of each pretreated and untreated rabbits. After 9 days tumour lesions affecting the inoculating area were noticed and the DI 50/0.5 of Shope's fibroma virus was then determined for each of the 4 experimental groups. The rabbits pretreated with T. spiralis exhibited much lower virus titres than the controls, which was evidently related to a certain degree of aspecific immunity conferred by the parasite. The results indicated that T. spiralis produces, in rabbits, resistance to Shope's fibroma virus and its neoplastic effect.

Immunosuppression during viral oncogenesis. V. Resistance to virus-induced immunosuppressive factor.

Rabbits given malignant rabbit fibroma virus (MV) develop severe immunologic dysfunction during the course of infection. Splenic T lymphocytes from these rabbits elaborate a soluble non-specific immunosuppressive factor (virus-induced suppressor factor (VISF]. As malignant rabbit fibroma virus infection progresses, normal immunologic responsiveness returns. This recovery is multi-factorial and involves production by T lymphocytes of a soluble factor capable of antagonizing the activity of VISF. This soluble anti-suppressor factor (ASF) is not a generalized immunologic potentiator. Its sole apparent effect on immune function appears to be to antagonize the activity of VISF. The protective effects of ASF are evident only when suppressor factors and ASF are simultaneously present in culture. Pre-treatment of target cells with ASF-containing culture supernatants does not render them insensitive to the immunosuppressive effects of subsequent treatment with VISF. In addition, ASF appears to be directly responsible for antagonizing VISF activity. That is, ASF does not appear to initiate an anti-suppressive cascade by activating a population of cells that in turn generate secondary protective factors. ASF-producing cells do not bind Vicia villosa lectin, as do contra-suppressor cells described by others. In almost all of these features, the system we describe herein differs from systems in which other investigators have described factors that antagonize the effects of suppressor factors.

Growth of malignant rabbit fibroma virus in lymphoid cells.

To understand better the immunosuppressive capacity of malignant rabbit fibroma virus (MV), we characterized MV growth in lymphoid cells. Replication of MV occurs in unstimulated normal spleen cells in vitro and is enhanced by adding T- or B-lymphocyte mitogens. In splenic T-lymphocyte preparations, comparable results are found: virus growth in the absence of mitogen, augmented by adding Con A. Unlike mature T cells, thymic lymphocytes support MV replication only when mitogen is added. When spleen cells from rabbits infected with MV in vivo are removed and cultured without mitogen, MV growth is again observed, with virus titer increasing about 10-fold per day of culture. In spleen cell populations from MV tumor-bearing rabbits, MV grows best in T lymphocytes, moderately in B lymphocytes, and least efficiently in adherent cells. When spleen cells are examined immediately following sacrifice, MV antigens are expressed solely on T lymphocytes from rabbits infected in vivo with MV 7 days previously. However, following overnight incubation in vitro a population of non-T lymphocytes displays cell membrane virus antigens. MV adapts itself somewhat to growth in lymphocytes, showing significantly greater growth in lymphocytes following passage in lymphocytes than is observed for non-lymphocyte-propagated virus. MV-infected lymphocytes also elaborate a factor that enhances MV growth in lymphocytes. Thus, MV replicates preferentially in mature T lymphocytes but will grow well in B cells as well. In vivo infection produces relatively small amounts of recoverable virus. However, when these lymphocytes are cultured in vitro virus replicates very well without added mitogens. These growth patterns may help to understand MV-induced immunologic dysfunction.

Virus-lymphocyte interactions during the course of immunosuppressive virus infection.

Malignant rabbit fibroma virus (MV) is a lymphocytotropic leporipoxvirus which produces profound immunological dysfunction and lethal fibromyxosarcoma. We examined virus recovery from splenic lymphocytes as a function of time after inoculation in vivo, and correlated this with both immunological function and expression of virus-induced host suppressor activity. MV was most abundant in lymphocytes obtained 4 days following inoculation. At that time, immune function was relatively normal and host suppressor activity was not observed. By 7 days after infection, when active host immunosuppressor functions were observed, virus recovery was decreased. Eleven days post-inoculation host immune function began to recover despite increasing virus-induced tumours and developing opportunistic infection. Simultaneously, MV was no longer recoverable from spleen cells. Spleen cells from day 11 tumour-bearing rabbits did not support MV replication as efficiently as did normal or day 4 or 7 splenic lymphocytes, but they did not alter the ability of MV to grow in the latter cells. By fluorescence examination and cytofluorography, splenic lymphocytes bearing MV antigens were abundant 7 days after infection but disappeared by 11 days. This was temporally related to production of neutralizing antibody to MV, and development of virus-specific lymphocyte proliferative activity. The composition of splenic lymphocytes changed as well: the normal ratio of about 1:1 for B and T cells changed to 1:2 by day 7, and then inverted to almost 2:1 by day 11. Rabbits infected with MV thus appear to recover their immune function, concurrently eliminate virus-infected lymphocytes, and elaborate high titres of neutralizing serum antibodies despite progressive infections and tumour development.

Inhibition of virus replication does not alter malignant rabbit fibroma virus-induced immunosuppression.

Malignant rabbit fibroma virus (MV) directly suppresses generation of antibody responses and mitogen induced T and B lymphocyte proliferation. We investigated whether this phenomenon required expression of the complete viral genome. Phosphonoacetic acid (PAA) inhibits poxvirus specific DNA polymerases. Adding PAA to cultures reduces both MV replication and mitogen-driven rabbit lymphocyte proliferation in a dose-dependent fashion. A dose of PAA adequate to inhibit MV replication by about 97%, but insufficient to reduce lymphocyte proliferation appreciably, does not affect the ability of MV to suppress lymphocyte proliferation or initiation of antibody production. Spleen cells from MV tumour-bearing rabbits contain very little virus, but inhibit the proliferative and antibody forming responses of normal spleen cells. This activity is shown here to reflect the production by T lymphocytes of a soluble mediator of greater than 25 kD molecular weight. Adding PAA to these mixed spleen cell cultures does not alter the ability of MV to induce T suppressor activity in host lymphocytes. Thus, these immunosuppressive capabilities of MV appear to reflect early MV gene functions.

Reversal of virus-induced immune suppression.

We studied the immunosuppressive capacity of splenic lymphocytes from rabbits at different stages of progressive myxosarcoma induced by malignant rabbit fibroma virus (MV). Spleen cells taken from rabbits 7 days after virus inoculation proliferate poorly in response to Con A, and suppress normal responses to the mitogen. Those from animals 11 days after virus injection have recovered partially from MV-induced suppression. Further, their Con A responses are no longer suppressed by day 7 spleen cells. Supernatants from cultures of spleen cells from rabbits given MV 7 days previously suppress both antibody-producing and proliferative responses to unrelated antigens. Comparable supernatants from rabbits receiving MV 11 days before sacrifice neither suppress nor augment such responses. Mixing cells from 7 or 11 day MV rabbits with normal spleen cells gives similar results. When supernatants from spleen cell of rabbits with tumors induced 7 and 11 days previously are mixed, the supernatants from rabbits with 11-day-old tumors inhibit the suppressive capacity of those from animals with 7-day-old tumors. Similarly, mixing spleen cells from rabbits given MV 7 and 11 days previously results in culture supernatants that do not suppress normal antibody and proliferative responses. The ability of cells from rabbits given MV 11 days before to inhibit the effects of cells from rabbits given MV 7 days previously does not involve the production of interferon. Thus, despite progressive tumor burden, immunologic recovery is observed in rabbits 11 days after tumor virus inoculation. One factor in this recovery may be the generation of active inhibitors of virus-induced immunosuppression. Similar mechanisms may apply to recovery of immunologic function in other virus infections as well.

Cross-reactivity among cowpox, ectromelia and vaccinia viruses with monoclonal antibodies recognizing distinct antigenic determinants in A-type inclusion bodies.

Several monoclonal antibodies recognizing distinct antigenic determinants in A-type inclusion bodies (ATIB) induced by cowpox virus (CPV) were obtained to examine the cross-reactivity among various strains of poxviridae, comprising CPV, ectromelia virus (EV), vaccinia virus (VV) and Shope fibroma virus (SFV). The monoclonal antibodies were classified into at least 3 groups on the basis of the results of an immunofluorescence test and immunoblotting; i. e., strain-specific, CPV and EV-specific and Orthopoxvirus (CPV, EV and VV)-specific antibodies. Differences were found between the antigenic determinants of ATIB of LB strains (LB red and LB white) and other strains (Amsterdam, 53, 58 and 60) of CPV and also between those of ATIB of CPV and EV. An interesting finding was that VV also produces the antigen analogous to that associated with ATIB in CPV- and EV-infected cells despite the absence of morphologically defined ATIB.

Strain differences in Shope fibroma virus. An immunopathologic study.

The pathogenic effects of plaque-purified Boerlage and Patuxent strains of Shope fibroma virus (SFV) in neonatal rabbits are compared with results of previous reports which used nonpurified SFV. Clinically, the Boerlage strain produced large tumors; whereas the same dose of Patuxent strain SFV induced much smaller tumors locally. Neither virus caused metastatic or extensively invasive local spread in our study. Some Patuxent recipients died of respiratory infections prior to sacrifice. However, both groups of rabbits handled the tumor well; the tumor began regressing 15-20 days after inoculation. Histologically, the tumors produced by those viruses were identical. Patuxent strain recipients were otherwise normal. Boerlage strain recipients showed increased persistence of extramedullary hematopoiesis and scattered foci of parenchymal necrosis in their livers. They also showed considerable cell death in thymic lobules. In rabbits given Patuxent strain SFV, virus antigens were detected only in the tumor by immunohistologic examination. Boerlage viral antigens were found in the tumor and overlying skin. We also detected virus systemically in Boerlage recipients: it was present in fixed tissue phagocytes in the spleen and liver and also in parenchymal cells of the lungs, liver, and kidney. Boerlage strain SFV recipients also showed detectable virus in their thymus, both at the periphery of the thymic lobules and in the connective tissue separating thymic lobules from each other. Despite the disseminated nature of the infection, rabbits that received the latter strain fared as well as those receiving Patuxent strain SFV.

Immunologic dysfunction during viral oncogenesis. II. Inhibition of cellular immunity to viral antigens by malignant rabbit fibroma virus.

The ability of two related viruses--Shope fibroma virus (SFV) and malignant rabbit fibroma virus (MV)--to induce virus-specific immune responses in lymphocytes of recipient animals was studied. SFV produces a benign local tumor which regresses in 12-14 days. Using an assay for virus-induced lymphocyte blastogenesis lymphocytes reactive to SFV were detected, both in rabbits bearing SFV-induced tumors and in rabbits whose SFV-induced tumor had regressed. These virus-reactive cells were detected in peripheral blood and spleen, and in lymph nodes draining the primary tumor. In contrast, MV produces a disseminated tumor and eventual death. MV does not induce detectable blastogenic responses in lymphocyte populations. SFV and MV are antigenically cross reactive: rabbits immune to SFV do not develop MV-induced tumors, and antisera to each virus neutralize both equally. Lymphocytes from SFV-infected rabbits proliferate in vitro in response to MV that has been inactivated by ultraviolet light (uv/MV) but not to infectious MV. In contrast, lymphocytes from rabbits infected with MV do not respond to uv-inactivated MV or to SFV. Thus, infectious MV inhibits the development of normal blastogenic responses in vivo and prevents the expression of those responses in lymphocytes from MV-resistant, SFV-immune rabbits in vitro. The relevance of this impairment to the differences in the clinical courses of SFV- and MV-induced tumors is discussed.

Immunologic dysfunction during viral oncogenesis. I. Nonspecific immunosuppression caused by malignant rabbit fibroma virus.

Malignant rabbit fibroma virus (MV) is a potent oncogenic poxvirus that produces a rapidly progressive syndrome of disseminated myxosarcoma, immunosuppression, and fatal gram-negative infection. MV is probably a recombinant between Shope fibroma virus (SFV) and rabbit myxoma virus, and is capable of preventing or aborting the in vitro proliferative responses of rabbit lymphocytes to B and T lymphocyte mitogens. Proliferative responses to sheep erythrocytes (SRBC) are similarly affected, although MV does not alter ongoing antibody responses to SRBC. Splenic lymphocytes from MV tumor-bearing rabbits suppress antibody and proliferative responses to SRBC when added to lymphocytes from SRBC-primed rabbits. Finally, lysates of cultured splenic lymphocytes from rabbits given MV suppress both proliferative and antibody-forming responses to SRBC. When MV is removed from these lysates by UV inactivation or by centrifugation, the suppressive activity remains. We therefore conclude that MV induces immunologic unresponsiveness in rabbits by at least two mechanisms. First, a direct suppressive effect of added virus on in vitro lymphocyte proliferation is seen. There is no effect in this situation if an antibody response is already in progress. Second, spleen cells exposed to MV in vivo produce one or more soluble factors capable of suppressing both proliferative and antibody responses of normal lymphocytes.

Immunohistology of malignant rabbit fibroma virus--a comparative study with rabbit myxoma virus.

Malignant rabbit fibroma virus (MV) causes a syndrome that consists of disseminated malignant tumors and immunosuppression complicated by severe Pasteurella multocida infection and death. Tissues from rabbits given MV and rabbit myxoma virus were examined by direct immunofluorescence with the use of antibody against virus antigens. Primary and metastatic tumors caused by MV and rabbit myxoma virus were composed of soft tissue cells containing virus antigens. Skin appendages and epidermis overlying the respective tumors showed scant MV but abundant myxoma virus antigen. Both viruses were present systemically in the reticuloendothelial system. Epithelial cells from the liver, kidney, and lung of myxoma virus-infected rabbits contained virus, whereas in MV tumor-bearing rabbits, these cells were uninvolved. However, nasal mucosal and conjunctival epithelia, the locations of Pasteurella infection, showed squamous metaplasia and contained large amounts of MV and myxoma antigens. By analogy to other respiratory tract pathogens, these epithelial changes were probably etiologically significant for development of pasteurellosis in rabbits bearing virus-induced tumors. Thus by immunopathologic as well as clinical examination, MV produces a syndrome distinct from that seen with rabbit myxoma virus. MV induced severe immunosuppression despite T-lymphocyte hyperplasia in the lymphoid tissues observed. The combination of a systemic virus infection, epithelial alterations that impaired clearance mechanisms, and immunologic dysfunction is likely to contribute to the inability of rabbits given MV to survive their gram-negative infection.

Immunity to pasteurellosis in compromised rabbits.

Pasteurellosis in the rabbit inoculated with a malignant variant of Shope fibroma virus (SFV-MV) is presented as a model for the study of immunosuppression and immunoprophylaxis in pasteurellosis. The rabbits, before the inoculation, were healthy carriers of Pasteurella multocida. They were intradermally inoculated with SFV-MV, and 3 to 6 days later, a primary tumor appeared at the site of inoculation. By postinoculation day (PID) 7 or 8, the rabbits had snuffles, conjunctivitis, and tumor metastases; death occurred on PID 10 to 14. Rabbits given the nonmalignant Patuxent strain of SFV developed local primary tumors, but not pasteurellosis nor metastases. In SFV-MV-inoculated rabbits, there was decreased responsiveness of spleen lymphocytes to B and T cell mitogens by day 6, and of spleen and peripheral blood lymphocytes by day 10. In addition, SFV-MV antigen was detected (by immunofluorescence) in mononuclear phagocytes in all major organs and in epithelial cells of the conjunctiva and nasal mucosa. Both nasal and conjunctival epithelia showed squamous metaplasia as well. These changes did not appear in SFV-infected rabbits. With SFV-MV-inoculated rabbits, we obtained partial protection against pasteurellosis by immunization with heat-killed P multocida or a cross-protective core lipopolysaccharide mutant of Escherichia coli (J5). Rabbits were immunized before the inoculation with SFV-MV which precipitated "spontaneous" pasteurellosis due to impaired defenses. Rabbits immunized with J5 or P multocida had less severe conjunctivitis and snuffles than nonimmunized controls, indicating that immunization with the J5 mutant may be useful as prophylaxis against pasteurellosis in compromised hosts.

Malignant rabbit fibroma virus causes secondary immunosuppression in rabbits.

Shope fibroma virus (SFV) causes a localized, self-limited, fibroblastic proliferation in adult rabbits. Extracts of Shope fibroma tumors were found to contain a second virus that induces a rapidly progressive disseminated tumor. Dissemination of this malignant fibroma is associated with activation of commensal mucosal infection with Pasteurella multocida, causing purulent conjunctivitis and rhinitis and resulting in death from nasal obstruction. We have isolated this new agent by two cycles of plaque purification. It is a poxvirus that is antigenically virtually identical to SFV as measured by a plaque reduction assay, but behaves differently both in vivo and in vitro. We have called this virus malignant rabbit fibroma virus (MV). Electrophoresis of restriction digests made with HIND III indicates that despite the antigenic similarity of SFV and MV, the locations of HIND III sites in the two viral genomes are quite different. These experiments have enabled us to determine that MV was present in small quantities in our initial uncloned stock of Patuxent strain SFV. Lymphocytes from rabbits bearing MV-induced tumors responded poorly to both B and T lymphocyte mitogens. This nonspecific immunologic dysfunction is evident at or before the time when metastases and Gram-negative infection develop, and it becomes more profound as the disease progresses. MV-induced tumors may provide a model for Gram-negative infection and decreased immunologic responsiveness associated with malignancies.

An immunohistologic study of Shope fibroma virus in rabbits: tumor rejection by cellular reaction in adults and progressive systemic reticuloendothelial infection in neonates.

The development of lesions in adult and neonatal New Zealand White rabbits following intradermal inoculation of Shope fibroma virus was studied by immunofluorescence for viral antigens. T-cells, and immunoglobulin. In adults a self-limiting local fibroxanthosarcomatous tumor was rejected within 10-12 days in association with a dense infiltration of T-cells. In neonates expanding skin lesions were associated with systemic presence of virus in the reticuloendothelial system. In surviving infected neonates, granulomas formed at the site of infection after 3 weeks. These reactions may have limited further dissemination of the virus. These results support the hypothesis that the progressive disease produced by Shope fibroma virus in neonatal rabbits may be due to the inability of the reticuloendothelial system to clear infectious virus.