RSV-induced Tumors Research Articles

Cytokine response to the RSV antigen delivered by dendritic cell-directed vaccination in congenic chicken lines

Systems of antigen delivery into antigen-presenting cells represent an important novel strategy in chicken vaccine development. In this study, we verified the ability of Rous sarcoma virus (RSV) antigens fused with streptavidin to be targeted by specific biotinylated monoclonal antibody (anti-CD205) into dendritic cells and induce virus-specific protective immunity. The method was tested in four congenic lines of chickens that are either resistant or susceptible to the progressive growth of RSV-induced tumors. Our analyses confirmed that the biot-anti-CD205-SA-FITC complex was internalized by chicken splenocytes. In the cytokine expression profile, several significant differences were evident between RSV-challenged progressor and regressor chicken lines. A significant up-regulation of IL-2, IL-12, IL-15, and IL-18 expression was detected in immunized chickens of both regressor and progressor groups. Of these cytokines, IL-2 and IL-12 were most up-regulated 14 days post-challenge (dpc), while IL-15 and IL-18 were most up-regulated at 28 dpc. On the contrary, IL-10 expression was significantly down-regulated in all immunized groups of progressor chickens at 14 dpc. We detected significant up-regulation of IL-17 in the group of immunized progressors. LITAF down-regulation with iNOS up-regulation was especially observed in the progressor group of immunized chickens that developed large tumors. Based on the increased expression of cytokines specific for activated dendritic cells, we conclude that our system is able to induce partial stimulation of specific cell types involved in cell-mediated immunity.

Genotypes of chicken major histocompatibility complex B locus associated with regression of Rous sarcoma virus J-strain tumors

The chicken MHC-B locus affects the response to several strains of Rous sarcoma virus (RSV). We evaluated the association between haplotypes of the MHC-B locus and responses to the J strain of RSV by using an F(2) experimental resource family constructed with tumor-regressive (White Leghorn) and tumor-progressive (Rhode Island Red) chickens. The MHC-B haplotypes were determined by genotyping of the microsatellite marker LEI0258 and MHC-B locus class I alpha chain 2 (BF2). Two haplotypes in the resource family, one associated with tumor regression and one with progression, were defined by these 2 markers. To discriminate more precisely the regressive haplotype in this family, we further developed 35 SNP markers at the MHC-B locus. Information on the haplotypes revealed here should be useful for identifying chickens with regression and progression phenotypes of J-strain RSV-induced tumors.

Quantitative Trait Loci Affecting Rous Sarcoma Virus Induced Tumor Regression Trait in F2 Intercross Chickens

We performed a genome-wide linkage and quantitative trait locus (QTL) analysis to confirm the existence of QTL affecting Rous Sarcoma Virus (RSV) induced tumor regression, and to estimate their effects on phenotypic variance in an F2 resource population. The F2 population comprised 158 chickens obtained by crossing tumor regressive White Leghorn (WL) and tumor progressive Rhode Island Red (RIR) lines was measured for tumor formation after RSV inoculation. Forty-three tumor progressive and 28 tumor regressive chickens were then used for genome-wide linkage and QTL analysis using a total of 186 microsatellite markers. Microsatellite markers were mapped on 20 autosomal chromosomes. A significant QTL was detected with marker LEI0258 located within the MHC B region on chromosome 16. This QTL had the highest F ratio (9.8) and accounted for 20.1% of the phenotypic variation. Suggestive QTL were also detected on chromosomes 4, 7 and 10. The QTL on chromosome 4 were detected at the 1% chromosome-wide level explaining 17.5% of the phenotypic variation, and the QTLs on chromosome 7 and 10 were detected at the 5% chromosome-wide level and explained 11.1% and 10.5% of the phenotypic variation, respectively. These results indicate that the QTLs in the non-MHC regions play a significant role in RSV-induced tumor regression. The present study constitutes one of the first preliminary reports in domestic chickens for QTLs affecting RSV-induced tumor regression outside the MHC region. I .

Protective Effects of Type I and Type II Interferons toward Rous Sarcoma Virus-Induced Tumors in Chickens

Growth of tumors induced by Rous sarcoma virus (RSV) is controlled by alleles at the major histocompatibility complex locus in chickens, indicating that immunological host defense mechanisms play a major role. We show here that the resistance phenotype of CB regressor chickens can be partially reverted by treating the animals with a monoclonal antibody that neutralizes the major serotype of chicken type I interferon, ChIFN-α. Injection of recombinant ChIFN-α into susceptible CC progressor chickens resulted in a dose-dependent inhibition of RSV-induced tumor development. This treatment was not effective, however, in CC chickens challenged with a DNA construct expressing the v-src oncogene, suggesting that the beneficial effect of type I interferon in this system resulted from its intrinsic antiviral activity and probably not from indirect immunmodulatory effects. By contrast, recombinant chicken interferon-γ strongly inhibited tumor growth when given to CC chickens that were challenged with the v-src oncogene, indicating that the two cytokines target different steps of tumor development.

Genes of chicken MHC regulate the adherence activity of blood monocytes in Rous sarcomas progressing and regressing lines

The influence of the chicken major histocompatibility (B) complex (MHC) on the adherence potential of monocyte-derived macrophages was examined using the congenic chicken lines CB and CC. These lines represent well-defined genetic models for the study of resistance (CB) or susceptibility (CC) to the progressive growth of Rous sarcomas. Using a monoclonal antibody specific for chicken monocytes/macrophages, CB and CC chickens were shown by flow cytometry analyses to have similar proportions of peripheral blood monocytes. However, when the glass-adherence potential of these cells was compared during incubation in tissue culture medium over 24, 48 and 72 h at 40°C, significant differences were seen between cells from these two inbred lines. After 24 and 48 h, glass-adherence by CB cells was 2–3 fold higher than that of CC cells. After 72 h this difference decreased to 1.5 fold. At 24 and 48 h, the adherent CB macrophages also appeared about 1.5 times larger than those of CC chickens. Genetic analysis using F 1 hybrids (CB×CC) showed that this trait is regulated by a dominant gene that segregates with the B12 haplotype in the backcross generation F 1×CC. From the results obtained with the recombinant congenic lines CB.R1 and CC.R1, we conclude that the gene regulating adherence potential is localized within the B-F/L region of the chicken MHC. About 50% of adherent cells were able to phagocytose opsonised FITC-labelled Zymosan particles. The level of nitric oxide production in vitro by CB and CC macrophages was equal. The importance of cells of the mononuclear phagocyte system for the response to Rous sarcoma virus (RSV) infection was studied in CB chickens using the anti-macrophage agents silica, carrageenan, and Cl 2MDP, encapsulated in liposomes. In those chickens treated with silica and carrageenan, we observed progressive growth of RSV-induced tumors. The graft-versus-host reactivity of peripheral blood lymphocytes (PBL) of treated chickens was comparable to controls. In vitro nitric oxide production by macrophages from silica-treated chickens was higher than by macrophages from untreated controls.

Src-specific immunity in inbred chickens bearing v-src DNA- and RSV-induced tumors.

The growth pattern (progression/regression) of v-src DNA- and Rous sarcoma virus (RSV)-induced tumors was analogous on a panel of inbred chicken lines. The decisive role of the major histocompatibility complex [Mhc(B)] alleles in resistance to the progression of these tumors was formally proved in segregating backcross populations. The immune mechanism of tumor regression was demonstrated by both in vivo and in vitro assays. A protective effect of v-src-specific immunity against RSV challenge was shown in Rous sarcoma regressor, line CB (B12/B12). Immune cells from regressors of v-src DNA-induced tumors can protect syngeneic hosts from the development of tumor after challenge with both v-src DNA and RSV. Suppression of RSV-induced tumor cell growth in vitro was also achieved by the use of cocultivation with spleen cells from chickens in which v-src DNA-induced tumors had regressed. This in vitro sarcoma-specific response was Mhc(B)-restricted. Chickens of the congenic Rous sarcoma progressor line CC (B4/B4) are sometimes able to regress v-src DNA-induced tumors, but immune cells can only slow the growth of v-src DNA-induced tumors in syngeneic hosts. This suggests that the primary reason for the susceptibility of CC chickens is a weak v-src-specific immune response. Furthermore, some of the v-src DNA-induced tumors were transplantable across the Mhc(B) barrier. The growth of tumor allografts was able to be facilitated when immunological tolerance to the B-F/L region antigens (class I and class II) had been established. This demonstrated that a high tumorigenicity of the transplantable tumor was not due to the lack of Mhc(B) antigens on tumor cells.

The Leukocyte Response of Japanese Quail to Rous Sarcoma Virus-Induced Tumors

In male Japanese quail, different circulating leukocyte responses were observed for progressors (birds developing a massive tumor that persisted throughout the experiment) and regressors (birds developing a tumor that gradually disappeared) after initial challenge with Rous sarcoma virus (RSV). Blood was sampled before and at weekly intervals postinoculation. Blood smears were prepared and stained with Diff Quik, and a light microscope (1000 x) was used in a direct count of 50 fields. Leukocytes were classified as heterophils, lymphocytes, monocytes, or eosinophils. The significant increase (P < 0.05) in total leukocytes at 14 days in regressors and progressors was consistent with the increase in tumor growth. The regressors' individual percentage of leukocytes did not deviate from control values, whereas the progressors' percentages of heterophils and monocytes were significantly higher (P < 0.05) and of lymphocytes significantly lower (P < 0.05) than those of controls by 14 days postinoculation. Indicative of this was the progressors' heterophil to lymphocyte ratio, which was significantly higher (P < 0.05) than that of controls 14 days post RSV challenge and remained elevated throughout the experiment. These findings suggest that the progressors' immune response is suppressed by proliferation of malignant cells. Therefore, the heterophil to lymphocyte ratio may be used in addition to tumor size to identify those birds that will regress RSV-induced tumors.

The 9E3 protein: immunolocalization in vivo and evidence for multiple forms in culture.

The avian gene 9E3/CEF4 belongs to a group of genes whose products are highly conserved and are homologous to inflammatory mediators. These genes, sometimes referred to as the gro family, are also expressed upon wounding or serum-stimulation of quiescent cells, suggesting that they may be important in aspects of growth and/or wound healing. We have used an antibody to the product of the 9E3 gene to show for the first time the distribution in vivo of the protein of one of these genes. The polyclonal antibody was produced against a synthetic peptide, [Cys76], 9E3, (77-103), located at the carboxy end of the molecule. The specificity of the antibody was determined by transfection of the 9E3 cDNA into Cos 7 cells, which do not express this gene. Moreover, despite the high homology between 9E3 and IL-8, the antibody did not crossreact with this molecule. The antibody was used to immuno-precipitate the protein from cultured normal and RSV-transformed chick embryo fibroblasts (CEFs) and to determine its distribution in tissues of newly hatched chicks. The staining was abundant in the cells and extracellular matrix (ECM) of connective tissue and other tissues of mesenchymal origin, such as bone and tendon. Most cells in the granulation tissue of wounds stained, some more intensely than others; the ECM also stained, especially in areas of scar tissue where collagen is abundant. In RSV-induced tumors, the protein was absent except in necrotic areas where a few cells--potentially macrophages--stained. In general, as expected, the protein was present in the cells and tissues that expressed the mRNA, but there were exceptions.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel effects of cytokines and hormones on APP stimulation

Growth of tumors induced by Rous sarcoma virus (RSV) is controlled by alleles at the major histocompatibility complex locus in chickens, indicating that immunological host defense mechanisms play a major role. We show here that the resistance phenotype of CB regressor chickens can be partially reverted by treating the animals with a monoclonal antibody that neutralizes the major serotype of chicken type I interferon, ChIFN-α. Injection of recombinant ChIFN-α into susceptible CC progressor chickens resulted in a dose-dependent inhibition of RSV-induced tumor development. This treatment was not effective, however, in CC chickens challenged with a DNA construct expressing the v-src oncogene, suggesting that the beneficial effect of type I interferon in this system resulted from its intrinsic antiviral activity and probably not from indirect immunmodulatory effects. By contrast, recombinant chicken interferon-γ strongly inhibited tumor growth when given to CC chickens that were challenged with the v-src oncogene, indicating that the two cytokines target different steps of tumor development.

Mechanism of target cell lysis by cytotoxic T lymphocytes (CTL) employing RSV-induced H-2 congenic and recombinant mouse tumor cells: Demonstration of soluble mediators released from H-2-restricted CTL clones

The secretion and the specificity of cytotoxic mediators from H-2-restricted cytotoxic T lymphocytes (CTL) were examined using non-virus-producing target tumor cells induced by the Schmidt-Ruppin strain of Rous sarcoma virus (SR-RSV) in B10 congenic and recombinant mice. By using rat concanavalin A supernatant, two H-2-restricted CTL clones were established from cytotoxic effector cells of B10.A(5R) mice primed with SR-RSV-induced syngeneic tumor cells. Cell-free supernatants from the H-2-restricted CTL clones cocultured with syngeneic tumor cells had selectively high cytotoxic activity for syngeneic and H-2-compatible tumor cells, but not for H-2-incompatible tumor cells, YAC-1 cells, and B10.A(5R) blasts as defined in the 5-hr 51Cr-release assay. The cytotoxic activity was detected in the cell-free supernatants from the CTL clones cocultured with the CTL-sensitive syngeneic and H-2-compatible tumor cells, but not with the CTL-insensitive tumor cells and YAC-1 cells. The cytotoxic activity of the cell-free supernatant could be adsorbed by the syngeneic tumor cells, but not by YAC-1 and L(s) cells. Thus, the H-2-restricted CTL clones against SR-RSV-induced tumor cells were capable of releasing cytotoxic mediators by coculturing with syngeneic or H-2-compatible tumor cells, and the cytotoxic mediators showed a certain H-2-restricted manner in killing the target cells. These results suggest that the lysis of RSV-induced tumor cells by H-2-restricted CTL can at least in part be mediated by cytotoxic factors.

No Expression of a Rous Sarcoma Virus-Induced Tumor Antigen in Mammalian Cells Infected With Retroviruses Transducing Other Oncogenes of the &lt;italic&gt;src&lt;/italic&gt; Gene Family&lt;xref ref-type="fn" rid="FN1"&gt;2&lt;/xref&gt;

Immunization with mouse and rat cells transformed by Rous sarcoma virus (RSV) or by B77 avian sarcoma virus (ASV) induced complete transplantation resistance against an RSV-induced mouse tumor (CSA1M) in syngeneic hosts. In contrast, most of the mice immunized with a Fujinami sarcoma virus-transformed rat fibroblast line (FSV-3Y1), a feline sarcoma virus-transformed cat fibroblast line (FeSV-FEF), an Abelson leukemia virus-infected Balb/3T3 cell line (AbLV-3T3), or an uninfected 3Y1 cell line could not reject the CSA1M. Serologic analysis with the use of a complement-dependent cytotoxicity assay supported the results of transplantation studies. The mouse and rat cells transformed by RSV or B77 ASV expressed a common tumor-specific cell surface antigen (TSSA) detected by syngeneic antiserum against the CSA1M, whereas none of the FSV-3Y1, FeSV-FEF, and AbLV-3T3 cells expressed the TSSA. These results suggest that the common TSSA in the mouse and rat cells transformed by RSV or B77 ASV containing src gene is not shared with mammalian cells infected with retroviruses transducing other oncogenes of the src gene family (i.e., fps, fes, and abl).

Independent Expression of Common and Private Tumor-Specific Cell Surface Antigens in Somatic Cell Hybrids Between Rous Sarcoma Virus-Transformed and Normal Cells&lt;xref ref-type="fn" rid="FN2"&gt;2&lt;/xref&gt;

Somatic cell hybrids were prepared by fusion of a Rous sarcoma virus (RSV)-induced mouse tumor (CSA1M cl-11) with a Chinese hamster embryo lung fibroblast line (Wg3h X AGr X Ouar). Some hybrid clones showed rapid anchorage-independent growth and intracellular production of RSV-pp60src. Other hybrid clones showed slow anchorage-dependent growth and no production of such a protein. The former expressed both a common tumor-specific cell surface antigen(s) (TSSA) shared with other RSV-induced tumors and a private TSSA not shared with any other tumors; however, the latter lost the common TSSA and expressed only the private TSSA. These results suggest that the common and private TSSA on CSA1M cl-11 cells are independent genetic traits and only the common TSSA is coordinated with other expressions of the RSV src gene phenotype.

Correlation between tumor-specific surface antigens and src gene expression in Rous sarcoma virus-induced rat tumors

Immunization with Rous sarcoma virus (RSV)-induced mouse tumors or with SR-3Y1 or NY8-3Y1 rat fibroblasts transformed by a wild type RSV or an env gene deletion mutant of RSV induced complete transplantation resistance against an RSV-induced mouse tumor (CSA1M) in syngeneic hosts. On the other hand, 10 of 19 mice immunized with ts68-3Y1 rat fibroblasts transformed by an src gene temperature-sensitive mutant of RSV (permissive temperature: 33–35° C) could not reject the CSA1M. SR-3Y1, NY8-3Y1 and RSV-induced mouse tumors expressed a common tumor-specific cell-surface antigen (TSSA) detected by a syngeneic rat antiserum against NY8-3Y1. In ts68-3Y1, expression of the TSSA was temperature-sensitive, TSSA being detected only when ts68-3Y1 was cultivated at the permissive temperature. Immunoprecipitation showed that serum from a rabbit bearing an RSV-induced tumor detected a 60- kdalton protein in cell extracts from SR-3Y1 and NY8-3Y1. Antiserum to NY8-3Y1 failed to detect this protein. These results suggest that the tumor-specific surface antigen(s) on RSV-induced mammalian tumors was coded for by an RSV src gene that was not identical with the simple form of the 60- kdalton protein identified.

Individually Distinct Tumor-Specific Cell Surface Antigen Identified by Monoclonal Antibody on a Rous Sarcoma Virus-Induced Mouse Tumor&lt;xref ref-type="fn" rid="fn2"&gt;2&lt;/xref&gt;

Hybrid cell lines were prepared by the fusion of BALB/c myeloma P3U-1 cells with the lymphocytes of BALB/c mice that were immunized with syngeneic Rous sarcoma virus (RSV)-induced tumor CSA1M cells. Three clones of the hybrid progeny (3.4B2, 3.4C6, and 3.5C11) produced cytotoxic IgM antibodies against CSA1M cells. One of the clones, 3.5C11, was chosen for analysis of the detailed specificity. Both direct cytotoxicity assays and absorption tests revealed that monoclonal antibody from 3.5C11 was positive only with CSA1M cells and that it failed to react with other tumors, including 20 RSV-induced mouse tumors, and normal cells. The 3.5C11 monoclonal antibody alone, with or without exogenous complement, was suppressive in the therapy of ip injected CSA1M tumor in syngeneic hosts, and significant prolongation in survival was seen in the treated mice. These results clearly showed presence of an individually distinct tumor-specific cell surface antigen on an RSV-induced mouse tumor.

Major Histocompatibility Complex (B): Effect on the Response of Chickens to a Second Challenge with Rous Sarcoma Virus

The influence of B genotype on the formation of a second RSV-induced tumor was studied. Line 63 (B2/B2) and (61 × 151) F3 and F4B2/B2 and B5 / B5 chickens were challenged with RSV in the left wingweb at 6 weeks of age. Tumor-bearing animals then were rechallenged with RSV in the right wingweb at either 10, 11, or 12 days post primary inoculation. Most B2/B2 hosts resisted development of a second tumor compared to none of the B5/B5 hosts. Failure of a second tumor to develop in B2/B2 hosts was not explained by humoral neutralization of RSV in vivo, since sera from 83% of B2/B2 and 100% of B5/B5 tumor bearing chickens failed to neutralize virus prior to 12 days post primary inoculation. Humoral antiviral immunity appeared later in B5/B5 than in B2/B2 hosts. Humoral antiviral immunity appears to play little if any role in resistance to early RSV-induced tumor development, but likely both humoral and cell-mediated antitumor immunity do.

Increased growth of RSV-induced tumors in chickens partially tolerant to MHC alloantigens.

Chickens with B2B2 MHC genotypes were made partically tolerant to B5 MHC cell-surface antigens and the fate of their Rous-sarcoma-virus (RSV)-induced tumors was determined. B2B2 chickens partially tolerant to viable or lysed white blood cells (WBC) or viable red blood cells (RBC) from B5B5 chickens had a significantly higher incidence of tumor progression than untreated, PBS-treated, or B2B2 chickens inoculated with WBC from other B2B2 chickens. The criteria for tolerance were absence of antibody titer to the cell type inoculated and acceptance of allografts from B5B5 donors by B2B2 chickens. Graft-vs-host reactions occurred only in B2B2 chickens inoculated with viable WBC from B5B5 chickens. It appears that B2B2 chickens partially tolerant to B5 antigens failed to mount a successful immune response to RSV-induced tumors partly because of a B5 MHC antigen(s) cross-reacted with a tumor associated antigen(s) thereby severely limiting B2B2 host recognition of the tumor as foreign. Since WBC and RBC cell-surface antigens appear to contribute similarly to the effect, the B-F- region of the MHC may be involved.

Host Age and Major Histocompatibility Genotype Influence on Rous Sarcoma Regression in Chickens ,

The influence of host age and B (major histocompatibility complex) genotype on the fate of Rous sarcoma virus (RSV)-induced tumors was investigated. White Leghorn lines 61 and 151, developed at the Regional Poultry Research Laboratory (RPRL) at East Lansing, MI, and their crosses (F3 to F5) segregating for the B genotype were used. Preliminary experiments with noninbred White Leghorns showed that chickens RSV-inoculated at 4 weeks of age had a significantly lower incidence of tumor regression than chickens RSV-inoculated at 6, 8, 10, 12, and 14 weeks of age. The (61 × 151)F5B2/B2 and B5/B5 chickens inoculated at 6 weeks of age had a lower incidence of tumor regression than (61 × 151)F3 and F4, B2/B2 and B5/B5 chickens RSV-inoculated between 58 and 88 weeks of age, respectively. Some (61 × 151) B5/B5 chickens inoculated between 58 and 88 weeks of age regressed their tumors while all chickens of corresponding genotype inoculated at 6 weeks of age died with tumors. The (61 × 151) B2/B2 chickens had a higher incidence of tumor regression than B5/B5 chickens regardless of age. The genetic effect of the major histocompatibility complex was relatively greater than that of host age at RSV-inoculation on the fate of RSV-induced tumors.

Mouse fibroblasts transformed by Rous sarcoma virus express a virus-specific non-virion transplantation antigen.

Rous sarcoma virus (RSV)-transformed fibroblasts from different animals species a serologically detectable virus-induced non-virion cell surface antigen (VCSA), whose expression is controlled by the transforming viral src gene and by a cellular gene. Now, by in vivo immunization, we have found that RSV-transformed fibroblasts from different mouse strains share a virus-specific transplantation antigen. In fact, only animals immunized with irradiated syngeneic or allogeneic fibroblasts transformed by RSV, but not animals immunized with cells transformed by different oncogenic agents, rejected a lethal dose of syngeneic RSV-induced tumor cells. Immunoprecipitation tests with monospecific antisera showed that the expression of this antigen did not correlate with the presence of intracellular viral proteins other than the src gene product pp60src. However, hyperimmunization of mice with hamster or quail fibroblasts transformed by RSV, that express a high level of pp60src, did not induce transplantation resistance. It is concluded that the expression of both the serologically-defined VCSA and the transplantation antigen is the result of the interaction of pp60src with host cell gene product(s) rather than the simple exposure of pp60src at the outer cell surface.

B-Haplotype Influence on Marek's Disease, Rous Sarcoma, and Lymphoid Leukosis Virus-Induced Tumors in Chickens

Experiments were conducted to establish whether B-haplotype genes that determine resistance to RSV-induced tumors would also influence resistance to Marek's disease and lymphoid leukosis. Matings of line 63 × 151 F3 or F4 breeders segregating for the Ea-B locus antigens that are markers for B2 and B5 haplotypes of the B major histocompatibility complex were made to produce B2B2, B2B5, and B5B5 chicks. The chicks were infected with a standard inoculum of one of the three tumor viruses in separate experiments. The B2 haplotype from 63 conveyed greater resistance to tumors induced by all three viruses than the B5 from 151 chickens. This finding suggests that some gene(s) in the B-haplotype may determine a general ability to resist tumor formation or cause tumor regression.

Cross-reactivity between RSV-induced tumor antigen and B5 MHC alloantigen in the chicken.

Lymphocytes from chickens homozygous (B2B2) at the major histocompatibility complex (MHC) were tested for cytotoxic against five types of target chicken embryo fibroblasts (CEF). Lymphocytes from B2B2 chickens bearing RSV-induced tumors lysed in vitro targets of B2B2 and B5B5 RSV-infected CEF and B5B5 normal CEF, but did not lyse B2B2 and B24B24 normal CEF. Lymphocytes from normal B2B2 chickens did not lyse any of the five types of CEF targets. Alloantisera absorption studies showed that both RSV-infected and uninfected CEF shared alloantigens, in particular B-F alloantigens, with syngeneic erythrocytes. Absorption with B2B2 RSV-infected CEF significantly lowered the titer of B2B2 anti-B5B5 alloantisera. Cross-reactivity between B5 antigen(s) and tumor-associated antigen was suggested and the nature of the cross-reactivity was discussed. It is hypothesized that this cross-reactivity prevents B5B5 chickens from recognizing the RSV-induced tumors as foreign, enhances tumor growth and leads to death of the host.