Spontaneous and induced herpesvirus genome expression in Marek's disease tumor cell lines

Temperature-induced reactivation of Marek's disease virus-transformed T cells ex vivo

Marek's disease virus (MDV) establishes latency in chicken T lymphocytes that can lead to T cell transformation and cancer. Transformed Marek's disease chicken cell lines (MDCCs) can be expanded ex vivo and provide a valuable model to study latency, transformation, and reactivation. Here, we developed MDCCs from chickens infected with MDV that fluoresce during lytic replication and reactivation. Sodium butyrate treatment increased fluorescent protein expression as evidenced by fluorescent microscopy, flow cytometry, and western blotting; however, it caused significant apoptosis and necrosis. Treatment of MDCCs by decreasing the temperature resulted in robust MDV reactivation without significant induction of apoptosis and necrosis. Furthermore, MDV reactivation was significantly affected by the time in culture that can affect downstream reactivation analyses. In all, our data show that fluorescent protein expression during reactivation is a robust tool to examine viral replication in live cells ex vivo, and temperature treatment is an efficient technique to induce reactivation without punitive effects on cell viability seen with chemical treatment.

The Importance of the Bursa of Fabricius, B Cells and T Cells for the Pathogenesis of Marek’s Disease: A Review

The importance of the bursa of Fabricius (BF) for the pathogenesis of Marek’s disease (MD) has been studied since the late 1960’s. In this review, the results of these studies are analyzed in the context of the developing knowledge of the immune system of chickens and the pathogenesis of MD from 1968 to 2022. Based on the available techniques to interfere with the development of the BF, three distinct periods are identified and discussed. During the initial period between 1968 and 1977, the use of neonatal bursectomy, chemical methods and irradiation were the main tools to interfere with the B lymphocyte development. The application of these techniques resulted in contradictory results from no effects to an increase or decrease in MD incidence. Starting in the late 1970’s, the use of bursectomy in 18-day-old embryos led to the development of the “Cornell model” for the pathogenesis of MD, in which the infection of B lymphocytes is an important first step in MD virus (MDV) replication causing the activation of thymus-derived lymphocytes (T cells). Following this model, these activated T cells, but not resting T cells, are susceptible to MDV infection and subsequent transformation. Finally, B-cell knockout chickens lacking the J gene segment of the IgY heavy chain gene were used to further define the role of the BF in the pathogenesis of MD.

Coinfection in the host can result in functional complementation between live vaccines and virulent virus

One of the greatest achievements of the last century is the development of vaccines against viral diseases. Vaccines are essential for battling infectious diseases and many different formulations are available, including live attenuated vaccines. However, the use of live attenuated vaccines has the potential for adverse effects, including reversion of pathogenicity, recombination, and functional complementation in the host. Marek’s disease is a serious disease in poultry controlled by live attenuated vaccines that has resulted in increased virulence over the decades. Recombination between circulating field viruses or vaccines is a proposed mechanism for the increase in virulence, however, complementation between vaccines and field strains has not been demonstrated in chickens. Here, we describe functional complementation of vaccines with virulent virus to functionally complement transmission and spread in the host. Using the natural virus-host model of Marek’s disease in chickens, our results show dual infection of target cells in chickens with vaccine and virulent virus providing the opportunity for recombination or complementation to transpire. Interestingly, our controlled results showed no evidence of recombination between vaccine and virulent virus, but functional complementation occurred in two independent experiments providing proof for complementation during natural infection in vaccinated individuals. These results suggest complementation as a potential mechanism for vaccine-mediated viral evolution and the potential for complementation should be taken into consideration when developing novel vaccines.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.

Expression of fluorescent proteins within the repeat long region of the Marek's disease virus genome allows direct identification of infected cells while retaining full pathogenicity

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus and causes Marek's disease (MD) in chickens. RLORF4 is an MDV-specific gene located in the repeat long (RL) regions of the genome and is directly involved in attenuation. In this report, we generated recombinant (r)MDVs in which eGFP or mRFP was inserted in-frame of the 3' end of the RLORF4 gene. In vitro growth was unaffected and infected cells could be identified by using fluorescent microscopy. Interestingly, though inserted in-frame with RLORF4, eGFP and mRFP were expressed alone, confirming mRNA expression and splicing within the RL of MDV is complex. In vivo, rMDVs expressing mRFP or eGFP caused tumors similar to wild-type MDV. Fluorescent protein expression could be seen in spleen, tumor, and feather follicle epithelial cells. These results show that expression of fluorescent proteins within the RL region results in fluorescent rMDVs that still maintains full pathogenicity in the chicken.

Detection of integrated herpesvirus genomes by fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) is widely used to visualize nucleotide sequences in interphase cells or on metaphase chromosomes using specific probes that are complementary to the respective targets. Besides its broad application in cytogenetics and cancer research, FISH facilitates the localization of virus genomes in infected cells. Some herpesviruses, including human herpesvirus 6 (HHV-6) and Marek's disease virus (MDV), have been shown to integrate their genetic material into host chromosomes, which allows transmission of HHV-6 via the germ line and is required for efficient MDV-induced tumor formation. We describe here the detection by FISH of integrated herpesvirus genomes in metaphase chromosomes and interphase nuclei of herpesvirus-infected cells.

Herpesvirus telomeric repeats facilitate genomic integration into host telomeres and mobilization of viral DNA during reactivation

Herpesvirus telomeric repeats facilitate virus integration into host telomeres, a process which is required for the establishment of virus latency.

Some herpesviruses, particularly lymphotropic viruses such as Marek’s disease virus (MDV) and human herpesvirus 6 (HHV-6), integrate their DNA into host chromosomes. MDV and HHV-6, among other herpesviruses, harbor telomeric repeats (TMRs) identical to host telomeres at either end of their linear genomes. Using MDV as a natural virus-host model, we show that herpesvirus TMRs facilitate viral genome integration into host telomeres and that integration is important for establishment of latency and lymphoma formation. Integration into host telomeres also aids in reactivation from the quiescent state of infection. Our results and the presence of TMRs in many herpesviruses suggest that integration mediated by viral TMRs is a conserved mechanism, which ensures faithful virus genome maintenance in host cells during cell division and allows efficient mobilization of dormant viral genomes. This finding is of particular importance as reactivation is critical for virus spread between susceptible individuals and is necessary for continued herpesvirus evolution and survival.

Herpesvirus telomerase RNA(vTR)-dependent lymphoma formation does not require interaction of vTR with telomerase reverse transcriptase (TERT)

Telomerase is a ribonucleoprotein complex involved in the maintenance of telomeres, a protective structure at the distal ends of chromosomes. The enzyme complex contains two main components, telomerase reverse transcriptase (TERT), the catalytic subunit, and telomerase RNA (TR), which serves as a template for the addition of telomeric repeats (TTAGGG)(n). Marek's disease virus (MDV), an oncogenic herpesvirus inducing fatal lymphoma in chickens, encodes a TR homologue, viral TR (vTR), which significantly contributes to MDV-induced lymphomagenesis. As recent studies have suggested that TRs possess functions independently of telomerase activity, we investigated if the tumor-promoting properties of MDV vTR are dependent on formation of a functional telomerase complex. The P6.1 stem-loop of TR is known to mediate TR-TERT complex formation and we show here that interaction of vTR with TERT and, consequently, telomerase activity was efficiently abrogated by the disruption of the vTR P6.1 stem-loop (P6.1mut). Recombinant MDV carrying the P6.1mut stem-loop mutation were generated and tested for their behavior in the natural host in vivo. In contrast to viruses lacking vTR, all animals infected with the P6.1mut viruses developed MDV-induced lymphomas, but onset of tumor formation was significantly delayed. P6.1mut viruses induced enhanced metastasis, indicating functionality of non-complexed vTR in tumor dissemination. We discovered that RPL22, a cellular factor involved in T-cell development and virus-induced transformation, directly interacts with wild-type and mutant vTR and is, consequently, relocalized to the nucleoplasm. Our study provides the first evidence that expression of TR, in this case encoded by a herpesvirus, is pro-oncogenic in the absence of telomerase activity.

Analysis of transcriptional and translational activities of Marek's disease (MD) virus genes in MD central nervous system lesions in chickens

The expression of Marek's disease virus (MDV) gene products and transcripts was observed in tissues obtained from the central nervous system (CNS) of chickens experimentally infected with a very virulent strain (Md/5) of MDV. Many cells antigen-positive for MDV early gene products, but not for late gene products, were detected immunohistochemically in the necrotizing lymphomatous lesions. The positive signals were found only in necrotic or degenerated neoplastic lymphoblasts. Abundant transcriptional activity of MDV genes was observed in the necrotizing lymphomatous lesions for immediate-early and early genes, but not for late ones, by the reverse-transcription polymerase chain reaction. In the non-necrotizing lymphomatous lesions, as well as non-suppurative ones, there were no or very few antigen-positive lymphoblasts for early and late genes, and little transcriptional activity of MDV genes. The necrotizing lesions of the lymphoma were associated with necrotizing vasculitis in which endothelial cells exhibited up-regulation of MHC class II antigen but no viral antigens. The present results indicate that only necrotizing lymphomatous lesions revealed abundant incomplete cytolytic cycle in MDV latently infected neoplastic lymphoblasts.

Effect of cyclosporin a on normal, mitogen-stimulated and Marek's disease virus-exposed and transformed chicken lymphoid cells

Studies have been made with the immunosuppressive drug cyclosporin A (CsA) to examine its value in the establishment of lymphoid tumour cell lines from Marek's disease (MD) lymphomas and from lymphoid cell cultures exposed to MD virus in vitro. CsA was shown to depress the proliferative response of normal spleen cells to phytohaemagglutinin, Concanavalin A and pokeweed mitogen, and to a lesser extent to lipopolysaccharide. Short-term proliferative responses of lymphoma cells were either not affected, depressed or stimulated by CsA. The efficiency of establishment of lymphoid cell lines from long-term cultures of lymphoma cells was not increased by CsA, and the drug had a depressive effect on the proliferation of cell lines in the lympho-cytoid stage. The majority of lymphoblastoid cell lines studied were stimulated by CsA. Interleukin 2 partially overcame the suppressive effect of CsA on the cell lines, and enhanced the stimulatory effects. Cultures of lymphoid cells exposed to MD virus in vitro were usually depressed by CsA; a few stimulatory combinations were observed, but these were not considered to be of biological significance. These results indicate that CsA suppresses normal T-cell responses in the chicken, but that some MD-associated lymphoid cells are stimulated by the drug, in some instances at least by a direct effect.

Modeling the proteome of a Marek's disease transformed cell line: a natural animal model for CD30 overexpressing lymphomas

Marek's disease (MD) in the chicken, caused by the highly infectious MD alpha-herpesvirus (MDV), is both commercially important and a unique, naturally occurring model for human T-cell lymphomas overexpressing the Hodgkin's disease antigen, CD30. Here, we used proteomics as a basis for modeling the molecular functions and biological processes involved in MDV-induced lymphomagenesis. Proteins were extracted from an MDV-transformed cell line and were then identified using 2-D LC-ESI-MS/MS. From the resulting 3870 cellular and 21 MDV proteins we confirm the existence of 3150 "predicted" and 12 "hypothetical" chicken proteins. The UA-01 proteome is proliferative, differentiated, angiogenic, pro-metastatic and pro-immune-escape but anti-programmed cell death, -anergy, -quiescence and -senescence and is consistent with a cancer phenotype. In particular, the pro-metastatic integrin signaling pathway and the ERK/MAPK signaling pathways were the two predominant signaling pathways represented. The cytokines, cytokine receptors, and their related proteins suggest that UA-01 has a regulatory T-cell phenotype.

Expression of Marek's disease virus phosphorylated polypeptide pp38 produces splice variants and enhances metabolic activity

The phosphorylated polypeptide (pp)38 of oncogenic Marek's disease (MD) herpesvirus (MDV) is expressed during lytic infections in vivo and in vitro, but its functions have not been fully elucidated. The quail cell line QT-35, latently infected with MDV, was used to generate QTP32 in which pp38 is expressed under control of a tetracycline controlled promoter to examine possible functions of pp38. Induction of pp38 did not influence late MDV genes expression, but it enhanced mitochondrial dehydrogenase activity significantly. Two new pp38 splice variants were found in induced QTP32 cells, in additional in vitro systems and MDV-infected chickens. Differential expression of full-length pp38 and splice variants suggests that the splice variants are important during latency and perhaps transformation. Polypeptides of 40 and 20kDa were detected by Western blot using monoclonal antibody H19. These polypeptides were also produced in DF-1 cells transfected with a pp38 construct in which the splice acceptor sites had been mutated. Our results add important new information to the role of pp38 in the pathogenesis of MD. The data suggest that pp38 and the two newly described splice variants may influence metabolic activity, which may have important consequences for the understanding of latency and tumor development.

Marek's disease virus reactivation from latency: changes in gene expression at the origin of replication

Marek's disease is a contagious lymphoma of chickens caused by Marek's disease virus (MDV). MDV replicates in chicken lymphocytes and establishes latency within and transforms chicken CD4+ T-cells. Transformed T-cells are seen as skin leukosis or as lymphomas in visceral organs. A major focus of our laboratory is the functional study of genes flanking the origin of replication. This origin (OriLyt) is contained within the repeats flanking the unique long (UL) region of the genome (IRL and TRL). To the left of this Ori are genes associated with MDV latent/transforming infection [1.8-kb RNA family, pp14, Meq), and to the right (UL) are genes associated with early stages of MDV lytic infection [BamHI-H-encoded protein (Hep), pp38/pp24, Mys]. During latency, MDV suppresses lytic gene expression and has evolved mechanisms for blocking the apoptosis of latently-infected CD4+ T-cells. Of the genes expressed during MDV latency and in the transformed cell, the Meq (Marek's EcoRI-Q-encoded protein) has been shown to block apoptosis and transactivate gene expression. Upon reactivation to lytic infection, we have found that splice variants of Meq predominate and that these forms lack several of the domains important to Meq trans-activation and trans-repression. We have found that rightward from the origin of replication, a family genes, including phosphoprotein 38 (pp38) are expressed during early stages of reactivation. Three separate open reading frames (Hep, Mys, and pp38) are encoded by distinct transcripts from this region. We are now determining the kinetics of expression of these transcripts and their relative abundance during reactivation.

Impact of deletions within the Bam HI-L fragment of attenuated Marek's disease virus on vIL-8 expression and the newly identified transcript of open reading frame LORF4

Marek's disease (MD) in chickens is caused by MD herpesvirus (MDV), which induces T cell lymphomas. The early pathogenesis of MDV infection is characterized by a primary infection in B lymphocytes followed by infection of activated T lymphocytes. It has been speculated that a MDV-encoded homologue of interleukin-8 (vIL-8) may be important to attract activated T lymphocytes to infected B lymphocytes. Recently, more virulent strains of MDV have emerged, named very virulent plus (vv+)MDV, that cause earlier and more prolonged cytolytic infections compared to less virulent strains. In this report, it was found that vIL-8 mRNA expression in vivo was increased in very virulent (vv) and vv+MDV strains compared to mild (m) and virulent (v) strains, and could not be detected in two attenuated MDV strains examined using very sensitive real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assays. In order to identify potential mechanisms for the increased vIL-8 mRNA expression in more virulent strains, and lack thereof in attenuated strains, the vIL-8 gene and putative promoter sequences upstream of the vIL-8 gene were compared from 10 different MDV strains, including attenuated derivatives. Only the JM-16 strain (both non-attenuated and attenuated) and attenuated 584A (584Ap80C) encoded a predicted vIL-8 gene sequence different from all other strains examined. Within the putative vIL-8 gene promoter sequence, there was little difference among the non-attenuated strains; however significant deletions were identified in the attenuated JM-16/p71, Md11 (R2/23), and 584Ap80C strains. Additionally, these deletions were located within a previously hypothetical open reading frame (ORF) named LORF4. Rapid amplification of cDNA ends identified a full-length transcript of LORF4 in the MDV-transformed lymphoblastoid cell line MSB-1, and deletions within this ORF caused truncated predicted proteins in 4 out of 6 attenuated MDV strains examined.

Development of strain-specific real-time PCR and RT-PCR assays for quantitation of chicken anemia virus

Chicken anemia virus (CAV) is a ubiquitous pathogen of poultry. A CAV specific TaqMan-based PCR and RT-PCR assay for real-time quantitation of viral load and relative quantitation of virus-specific transcript levels was developed. Detection of viral DNA copy number from infected MDCC-CU147 cells was determined by extrapolation from a CAV plasmid-based standard curve. Viral load increased proportionally with increasing cell number harvested, increasing from 4x10(2) copies in 250 cells with 38% virus positive cells in an indirect immunofluorescence assay to 8x10(5) copies in 250,000 cells with 64% infected cells. The estimated average viral copy number per infected cell ranged from 5 to 14. Strain-specific primers were developed to distinguish between the Cux-1 and CIA-1 strains of CAV. These primers exhibited a 3 to 4 log differential in amplification comparing homologous versus heterologous virus-primer combinations. The sensitivity of the real-time assay was found to be comparable to a nested PCR assay using DNA samples from a SPF poultry flock exposed to the SH-1 strain of CAV. The real-time PCR detected from 1.7 to 4.2 target molecules in three out of four samples that were positive by nested PCR using 50% of the DNA used in the nested PCR. Relative viral transcript levels for Cux-1 and CIA-1 infected cell cultures increased proportionally with increasing cell numbers harvested for RNA extraction. This assay will be important for both diagnosis and in understanding the complex pathogenesis of CAV infection.

Transactivation of latent Marek's disease herpesvirus genes in QT35, a quail fibroblast cell line, by herpesvirus of turkeys

The QT35 cell line was established from a methylcholanthrene-induced tumor in Japanese quail (Coturnix coturnix japonica) (C. Moscovici, M. G. Moscovici, H. Jimenez, M. M. Lai, M. J. Hayman, and P. K. Vogt, Cell 11:95-103, 1977). Two independently maintained sublines of QT35 were found to be positive for Marek's disease virus (MDV)-like genes by Southern blotting and PCR assays. Sequence analysis of fragments of the ICP4, ICP22, ICP27, VP16, meq, pp14, pp38, open reading frame (ORF) L1, and glycoprotein B (gB) genes showed a strong homology with the corresponding fragments of MDV genes. Subsequently, a serotype 1 MDV-like herpesvirus, tentatively name QMDV, was rescued from QT35 cells in chicken kidney cell (CKC) cultures established from 6- to 9-day-old chicks inoculated at 8 days of embryonation with QT35 cells. Transmission electron microscopy failed to show herpesvirus particles in QT35 cells, but typical intranuclear herpesvirus particles were detected in CKCs. Reverse transcription-PCR analysis showed that the following QMDV transcripts were present in QT35 cells: sense and antisense meq, ORF L1, ICP4, and latency-associated transcripts, which are antisense to ICP4. A transcript of approximately 4.5 kb was detected by Northern blotting using total RNA from QT35 cells. Inoculation of QT35 cells with herpesvirus of turkeys (HVT)-infected chicken embryo fibroblasts (CEF) but not with uninfected CEF resulted in the activation of ICP22, ICP27, VP16, pp38, and gB. In addition, the level of ICP4 mRNA was increased compared to that in QT35 cells. The activation by HVT resulted in the production of pp38 protein. It was not possible to detect if the other activated genes were translated due to the lack of serotype 1-specific monoclonal antibodies.

Electron microscopic and immunohistochemical localization of Marek's disease (MD) herpesvirus particles in MD skin lymphomas

Skin lymphomas induced in 11 specific-pathogen-free chickens by inoculation at 1 day of age with Marek's disease virus (MDV) were biopsied weekly and examined by electron microscopy and immunohistochemistry. In the sequentially biopsied lymphomas, immature MDV particles (abortive replication) were found only in the nuclei of necrotic lymphoblasts within necrotizing neoplasms. The necrotizing lymphomas were observed in two of the 11 experimental birds and were associated with prominent vascular endothelial cell injury, including fibrinoid necrosis of blood vessels. Nonnecrotizing lymphomas biopsied sequentially from the 11 experimental birds did not contain virus particles of any kind in the lymphoblasts and had no distinct vascular lesions. Immunohistochemically, MDV early antigen (pp38), but not late antigens (glycoproteins B and C), was detected only in the necrotizing lymphomas. These findings indicate that abortive MDV replication mainly occurred in necrotic lymphoblasts, which might have been induced by ischemia.

Recombinant Marek's disease virus (MDV)-derived lymphoblastoid cell lines: regulation of a marker gene within the context of the MDV genome

Marek's disease is a herpesvirus (Marek's disease virus [MDV])-induced pathology of chickens characterized by paralysis and the rapid appearance of T-cell lymphomas. Lymphoblastoid cell lines (LBCLs) derived from MDV-induced tumors have served as models of MDV latency and transformation. We have recently reported the construction of mutant MDVs having a deletion (M. S. Parcells et al., J. Virol. 69:7888-7898, 1995) and an insertion (A. S. Anderson et al., J. Virol. 72:2548-2553, 1998) within the unique short region of the virus genome. These mutant MDVs retained oncogenicity, and LBCLs have been established from the mutant-induced tumors. We report the characterization of these cell lines with respect to (i) virus structure within and reactivated from the cell lines, (ii) surface antigen expression, (iii) kinetics of MDV and marker gene induction, (iv) localization and colocalization of induced MDV antigens and beta-galactosidase (beta-Gal), and (v) methylation status of the region of lacZ insertion in recombinant- and non-recombinant-derived cell lines. Our results indicate that (i) recombinant-derived cell lines contain no parental virus, (ii) the established cell lines are predominantly CD4(+) CD8(-), (iii) the percentage of Lac-expressing cells is low (1 to 3%) but increases dramatically upon 5'-iododeoxyuridine (IUdR) treatment, (iv) lacZ expression is induced with the same kinetics as several MDV lytic-phase genes (pp38, US1, gB, gI, and US10), and (v) the regulation of lacZ expression is not mediated by methylation. Furthermore, the MDV-encoded oncoprotein, Meq, could be detected in cells expressing beta-Gal and various lytic antigens but did not appear to be induced by IUdR treatment. Our results indicate that regulation of the lacZ marker gene can serve as sensitive measure of virus lytic-phase induction and the reactivation from latency.

Transforming potential of the herpesvirus oncoprotein MEQ: morphological transformation, serum-independent growth, and inhibition of apoptosis

Marek's disease virus (MDV) induces the rapid development of overwhelming T-cell lymphomas in chickens. One of its candidate oncogenes, meq (MDV Eco Q) which encodes a bZIP protein, has been biochemically characterized as a transcription factor. Interestingly, MEQ proteins are expressed not only in the nucleoplasm but also in the coiled bodies and the nucleolus. Its novel subcellular localization suggests that MEQ may be involved in other functions beyond its transcriptional potential. In this report we show that MEQ proteins are expressed ubiquitously and abundantly in MDV tumor cell lines. Overexpression of MEQ results in transformation of a rodent fibroblast cell line, Rat-2. The criteria of transformation are based on morphological transfiguration, anchorage-independent growth, and serum-independent growth. Furthermore, MEQ is able to distend the transforming capacity of MEQ-transformed Rat-2 cells through inhibition of apoptosis. Specifically, MEQ can efficiently protect Rat-2 cells from cell death induced by multiple modes including tumor necrosis factor alpha, C2-ceramide, UV irradiation, and serum deprivation. Its antiapoptotic function requires new protein synthesis, as treatment with a protein synthesis inhibitor, cycloheximide, partially reversed MEQ's antiapoptotic effect. Coincidentally, transcriptional induction of bcl-2 and suppression of bax are also observed in MEQ-transformed Rat-2 cells. Taken together, our results suggest that MEQ antagonizes apoptosis through regulation of its downstream target genes involved in apoptotic and/or antiapoptotic pathways.

Nitric oxide synthase activity and mRNA expression in chicken macrophages

The activity of inducible nitric oxide synthase (iNOS) enzyme was quantified in chicken macrophages. Macrophages from Cornell K-strain (B15B15), GB1 (B13B13), and GB2 (B6B6) chickens and a transformed cell line (MQ-NCSU) were incubated with or without varying concentrations of bacterial lipopolysaccharide (LPS). The culture supernatants were tested for the presence of nitrite. Macrophages from either source produced minimal nitrite (< 4.4 microM/1 x 10(6) cells) levels without LPS stimulation. However, nitrite levels produced by K-strain (42 microM) and MQ-NCSU (41 microM) macrophages were higher (P < 0.05) than those produced by the GB1 (14 microM) and GB2 (14 microM) per 1 x 10(6) macrophages with optimum LPS concentration range of 50 ng to 1 microgram/mL. The addition of an L-arginine analog, NGMMLA, at a concentration of 200 microM completely abolished nitrite production. The addition of 10% vol/vol lymphokines exhibited an additive effect on nitrite production in conjunction with LPS. The increased nitrite production by the K-strain and MQ-NCSU macrophages corresponded to an increased expression of iNOS mRNA as compared to the mRNA produced by GB1 and GB2 macrophages. The iNOS mRNA kinetics study revealed that mRNA levels peaked between 6 to 12 h. The cells from avian lymphoid lineage failed to produce any detectable iNOS activity. These studies showed that macrophages from varying sources differ in NOS activity and implied that genetic background may dictate the extent of arginine-mediated contribution in various biological and immunological functions.

A hypervariable region in VP1 of chicken infectious anemia virus mediates rate of spread and cell tropism in tissue culture

Chicken infectious anemia virus (CIAV) is a unique infectious agent with an amino acid composition that has been found to be remarkably conserved even in isolates from different parts of the world. We have characterized field isolates of CIAV which vary significantly in terms of their abilities to replicate in culture, demonstrating a biological difference between isolates. Two sublines of MDCC-MSB1 cells that differ in their abilities to support CIAV were identified. In the MSB1(S) subline the CIA-1 isolate of CIAV was found to be less cytopathogenic than the prototype Cux-1(C) isolate; the MSB1(L) subline, which supports Cux-1(C) replication, was found to be nonpermissive for CIA-1. Alignments of the VP1 sequences of previously examined isolates with those of the field isolates CIA-1 and L-028 and the culture-adapted ConnB isolate revealed a previously unreported hypervariable region spanning amino acid positions 139 to 151. Chimeras of Cux-1(C) and CIA-1 were constructed to examine the potential for this region to affect cytopathogenicity. Transfer of a 316-bp region of Cux-1(C) open reading frame 1 into CIA-1 produced a virus with a cytopathogenic profile typical of Cux-1(C), indicating that one or both of the amino acid differences at positions 139 and 144 affect the rate of replication or the spread of infection. Transfection experiments with additional chimeras indicated that the inability of CIA-1 to replicate in MSB1(L) cells is mediated by a larger region of the genome which contains the hypervariable region in addition to upstream amino acid differences. Analysis of chimeras excluding the entire region of open reading frame 1 suggested the presence of a secondary mediator in the progression of infection in culture that was localized to a region containing a single nucleotide difference which results in amino acid differences in both VP2 (V-153) and the nuclear localization signal of VP3 (C-118). Immunofluorescence assays indicated an increased cytoplasmic distribution of VP3 and a general lack of VP3-associated apoptotic bodies in infections of CIA-1 and chimeras containing V-153 or C-118, as opposed to a primarily nuclear distribution and association with well-formed apoptotic bodies in Cux-1(C)-infected cells.

Stages of Marek's disease virus latency defined by variable sensitivity to interferon modulation of viral antigen expression

Cytokines in conditioned medium can suppress expression of viral internal antigens (VIA) in lymphocytes latently infected with Marek's disease virus. In the present study, conditioned media produced by spleen cells stimulated with concanavalin A or by mixed-lymphocyte reaction had significantly greater (P < 0.05) VIA-suppressive activity with lymphocytes harvested from birds at 14 days post infection than with those collected at 7 days. This finding defines two stages during the latent period in which sensitivity of lymphocytes to cytokine modulation of viral expression differs. Suppression involved proteins representing immediate-early, early and late viral antigens. Physico-chemical characterization of the suppressive factor in conditioned medium was consistent with that expected of interferon. Indeed, natural interferon prepared from avian reovirus-exposed chicken embryo cells, and recombinant chicken interferon, both mimicked the activity of conditioned medium and were more suppressive with lymphocytes from the later stage of latency.

The transcripts from the sequences flanking the short component of Marek's disease virus during latent infection form a unique family of 3'-coterminal RNAs

We have constructed a cDNA library using poly(A)+ RNA from the stably transformed Marek's disease virus cell line MKT-1 and isolated cDNAs specific to the short internal repeat region of the BamHI-A fragment of the viral genome. Four distinct classes of cDNA were identified through sequence analysis of the 5' and 3' termini of each clone isolated, and a representative of each class was chosen for complete sequencing. These cDNAs were mapped on the basis of the genomic nucleotide sequence of this region, and a family of 3'-coterminal overlapping transcripts consisting of several highly spliced species, was identified. PCR was used to amplify specific regions of each cDNA, which were subcloned and used to generate riboprobes. These riboprobes hybridized to a variety of transcripts in poly(A)+ RNA fractions isolated from cells either lytically or latently infected with Marek's disease virus.

Identification of latency-associated transcripts that map antisense to the ICP4 homolog gene of Marek's disease virus

Two small RNAs (0.9 and 0.75 kb), named Marek's disease virus (MDV) small RNAs (MSRs) and a 10-kb RNA, all of which map antisense to the MDV ICP4 homolog gene, have been readily detected in MDCC-MSB1 MDV-transformed T-lymphoblastoid cells. These RNAs were not detectable in reticuloendotheliosis virus-transformed T cells. When MDV was reactivated by treatment of lymphoblastoid cells with 25 micrograms of iododeoxyuridine per ml, the relative levels of the transcripts decreased. These RNAs were not detected by Northern (RNA) hybridization in productively infected chicken embryo fibroblasts 48 h postinfection; however, they were apparent 140 h postinfection. By using Northern hybridization, RNase protection assays, and primer extension analysis, the MSRs were determined to map antisense to the predicted translational start site of the ICP4 homolog gene. The conclusion most consistent with the data is that the two MSRs are overlapping, spliced RNAs. Both small RNAs contain a latency promoter binding factor consensus recognition sequence located toward their 5' ends as well as two potential ICP4 recognition consensus sequences, one in each orientation. The region contains a number of small open reading frames on each side and within the MSRs. Although the exact endpoints are unknown, the large 10-kb species spans the entire ICP4 homolog region. We believe that this group of RNAs, which map antisense to the ICP4 homolog gene, are latency-associated transcripts of MDV.

Analysis of macrophage functions in Rous sarcoma-induced tumor regressor and progressor 6.B congenic chickens

Macrophage functional competence was studied in two congenic chicken lines 6.6-2 (B2B2) and 6.15-5 (B5B5) which are regressors and progressors, respectively, of Rous sarcoma-induced tumors. Sephadex-elicited abdominal exudate cells (AEC) were harvested from 4-week-old chickens to determine their total number, glass adherence potential, percentage of adherent macrophages and phagocytosis of antibody coated (ops) and uncoated (unops) sheep red blood cells (SRBC). Tumoricidal abilities of culture medium conditioned with lipopolysaccharide treated macrophages and of macrophages cocultured with target cells were assessed against 51Cr-labelled tumor cell targets. The congenic lines did not differ in total AEC or percent macrophages. However, AEC from B5B5 birds exhibited significantly lower (P < 0.05) glass-adherence potential than AEC from B2B2 birds exhibited significantly lower (P < 0.05) glass-adherence potential than AEC from B2B2 birds. The percentage of phagocytic macrophages did not differ between lines for unop-SRBC, whereas a higher percentage of B5B5 compared with B2B2 birds (P < 0.05) macrophages phagocytized ops-SRBC. Macrophages from B5B5 birds had significantly (P < 0.05) lower activity in both tumoricidal tests. These results imply that the tumor progression in B5B5 birds is associated with reduced activation of macrophages towards a tumoricidal pathway.

Characterization of stimulatory surface molecular expression on avian T cells

We have characterized a monoclonal antibody (AT389), produced by immunization of mice with 4-day-old avian Concanavalin A blasts, which recognizes a 38-40 kDa homodimer present on the surface of all chicken T cells. AT389 increased the spontaneous and IL-2-driven proliferation of 3-day-old T-cell blasts. This antibody, in synergy with IL-2, increased cellular proliferation and cell survival of T-cell blasts over time. Together, these data suggest that AT389 recognizes a surface protein on avian T cells which may be related to known stimulatory surface markers on mammalian T cells.

Cell proteins bind to sites within the 3' noncoding region and the positive-strand leader sequence of measles virus RNA

The genomic 3' noncoding region (NCR) of nonsegmented negative-strand RNA viruses contains recognition site(s) for the polymerase complex, while the RNA plus-strand leader sequence (LS) is probably involved in RNA encapsidation. It is known that host-encoded factors play a role in transcription and replication of some of this group of viruses. Here we report that cellular proteins interact with the genomic 3' NCR and with the plus-strand LS RNA of an important human pathogen, measles virus (MV), a member of the family Paramyxoviridae. Using gel retardation assay and RNA footprinting analysis, we demonstrated that in Vero cells, host-encoded proteins bind specifically to domains within these two sequences. A polypeptide of about 20 kDa binding to the 3' NCR and two polypeptides of about 22 and 30 kDa interacting with plus-strand LS were detected by RNA-protein UV cross-linking. Different RNA-binding activities were found in cells differing in permissiveness to MV replication. The results suggest a role for host-encoded proteins in MV replication.

Status of Marek's disease virus in established lymphoma cell lines: herpesvirus integration is common

Six cell lines derived from Marek's disease lymphomas of chickens and turkeys were investigated for the status of Marek's disease virus (MDV) DNA. In the transformed T- and B-cell lines, viral DNA could be detected by conventional Southern blot hybridization, by Gardella gel electrophoresis, and by in situ hybridization of metaphase and interphase chromosomes. Integration of viral DNA into the host cell chromosome was observed in all cell lines. Two to 12 integration sites of viral DNA could be detected in metaphase chromosome spreads. The integration sites were characteristic for the individual cell lines and were preferentially located at the telomers of large- and mid-sized chromosomes or on minichromosomes. In four of six cell lines, a minor population of latently infected cells supported the lytic cycle of MDV, giving rise to linear virion DNAs. In one of these cell lines, a third species of MDV DNA could be detected with properties reminiscent of covalently closed circular DNA. The finding that MDV integrates regularly into the genomes of latently infected cells is crucial to understanding the molecular biology of herpesvirus-induced tumors in the natural host.

Characterization of reticuloendotheliosis virus-transformed avian T-lymphoblastoid cell lines infected with Marek's disease virus

The expression of Marek's disease virus (MDV) transcripts and protein products was investigated in reticuloendotheliosis virus-transformed avian T-lymphoblastoid cell line RECC-CU91, which was superinfected with MDV. The presence of MDV in the superinfected cell line, renamed RECC-CU210, was demonstrated by Southern hybridization with 32P-labeled BamHI-H and -B fragments of the BamHI MDV DNA library. Examination of RECC-CU210 for the expression of MDV-specific RNA transcripts encoded by the internal repeat long (IRL), internal repeat short (IRS), and unique short (US) regions of the MDV genome revealed two small transcripts of 0.6 and 0.7 kb. These transcripts were mapped to the IRL and IRS regions, respectively. In contrast, RECC-CU211, which was developed through transfection of CU210 with the BamHI-A fragment of MDV, expressed an additional nine transcripts from the IRL, IRS, and US regions. CU211 but not CU210 also expressed a complex of polypeptides of 40, 38, and 24 kDa, identified by monoclonal antibodies as MDV-specific phosphoproteins. The 38-kDa phosphoprotein is likely to be pp38, an early viral protein that maps within the IRL region of the MDV genome. These findings suggest that genes located within the transfected BamHI-A fragment transactivated a number of genes located in the IRL region of the MDV genome.

Marek's disease virus-transformed chicken T-cell lines respond to lymphokines

Current assays for chicken interleukin-2 (IL-2) utilize mitogen-activated lymphocytes. However, very high inter-assay variability and sporadic high background proliferation limit their usefulness. In view of the above, several Marek's disease virus (MDV)-transformed T-cell lines (which grow well in a serum-supplemented medium) were tested for a response to chicken IL-2 when grown in serum-free media. Five of six lines examined showed a dose-dependent proliferative response to chicken T-cell conditioned media. One line, MDCC-CU14, was chosen for further studies. In addition to the tumor cells' dose-dependent responses to semi-purified chicken IL-2, they expressed T-cell activation antigens on the cell surface. Furthermore, the level of surface expression was enhanced on cells provided IL-2. Co-incubation of the tumor cells with monoclonal antibody INN-CH-16 (specific for an antigen on the surface of activated T-cells) and IL-2 resulted in a modulation of lymphokine-induced proliferation. Together, these data suggest that signalling mechanisms in MDV T-cell tumors are intact and that these lines can be used as an assay for chicken T-cell lymphokines. Furthermore, they provide an interesting model for the study of avian and mammalian T-cell transformation. Implications for the study of Marek's disease are also discussed.

Complete nucleotide sequence of the Marek's disease virus ICP4 gene

The Marek's disease virus (MDV) gene encoding a homologue to the ICP4 protein of herpes simplex virus has been mapped to BamHl fragment A based on the physical map of the MDV genome (Fukuchi et al., 1984). The gene lies completely within the inverted repeat flanking the unique short region of the genome. The complete nucleotide sequence of the MDV ICP4 gene has been determined. The coding region is 4245 nucleotides long and has an overall G+C content of 52%. The MDV ICP4 protein is predicted to have a structure similar to that of ICP4-like proteins of other herpesviruses in that it has five distinct regions, the second and fourth of which are highly conserved. In addition, the protein contains the characteristic run of serine residues located toward its amino terminus. The MDV ICP4 gene is expressed in MDV-infected chicken embryo fibroblasts.

Molecular changes associated with heat-shock treatment in avian mononuclear and lymphoid lineage cells

The induction of heat-shock protein (HSP) synthesis in avian cells of the mononuclear phagocytic system (MPS) and lymphoid system (LS) lineage was investigated by exposure to in vitro heat-shock conditions. In addition, the kinetics of HSP90 mRNA expression was examined in chicken peritoneal macrophages (PM) as well as heat-shock-induced HSP synthesis in PM from chickens, turkeys, quail, and ducks. Each MPS and LS cell type expressed three major (23, 70, and 90 kDa) HSP following a 1-h heat shock at 45 C. However, a unique heat-induced 32-kDa protein (P32) was expressed only by cells of MPS lineage. The expression of HSP90 mRNA in chicken PM was temperature- and time-dependent. These findings imply that avian PM undergo molecular changes in response to elevated environmental temperatures and that the pattern of HSP expression appears to be distinct for cells of the MPS and LS lineages in chickens.

Increased ratio of targeted to random integration after transfection of chicken B cell lines

Constructs of four different genetic loci were transfected into the avian leukosis virus-induced chicken B cell line DT40, which continues diversification of its rearranged light chain immunoglobulin gene by gene conversion. Analysis of stable transfectants revealed an unexpectedly high frequency of targeted integration into the homologous gene loci of DT40. Transcriptional activity of the target gene locus is not required, since a construct of the untranscribed ovalbumin gene also integrated predominantly by homologous recombination. A construct derived from the beta-actin locus was transfected into other chicken cell lines to determine the cell type specificity of the phenomenon. Targeted integration still occurred at high frequency in two other B cell lines that do not have the gene conversion activity. However, the ratios of targeted to random integration were reduced by at least one order of magnitude in three non-B cell lines.

Signal requirements for the acquisition of tumoricidal competence by chicken peritoneal macrophages

The present study was conducted to determine the tumoricidal potential of chicken macrophages. Sephadex-elicited peritoneal macrophages from 6-wk-old Cornell K-strain White Leghorn females (B15B15) were used as effector cells against three different 51Cr-labeled tumor cell targets: LSCC-RP9 (B15B2), MDCC-CU14 (B19, C2), and MDCC-CU25 (B17B17). Quantification of tumoricidal activity was done in a 16-h, Cr-release assay. Macrophages collected at 12, 24, and 42 h post-Sephadex stimulation failed to kill any of the tumor cell targets. However, macrophages treated with concanavalin A stimulated splenic cell supernatants (lymphokines, LK) and lipopolysaccharide (LPS) were able to kill all three tumor cell targets in coculture experiments. Cell-free supernatants collected from LPS and LK alone or combination-treated macrophages demonstrated cytolytic activity for both RP9 and CU25 tumor cell targets. The results of the study, therefore, suggest that chicken macrophages acquire tumoricidal competence if treated with macrophage activation signals such as LK, or LPS or both.

Transformation of T-lymphocyte subsets by Marek's disease herpesvirus

Marek's disease herpesvirus (MDV)-transformed lymphoblastoid tumor cell lines were characterized for the presence of the surface markers. Monoclonal antibodies were used for CD3 (T-cell receptor [TCR] complex), TCR1, TCR2, and TCR3, CD4, CD8, and Ia antigen by indirect fluorescence staining followed by microscopic examination or flow cytometry. The lymphoblastoid cell lines were obtained from tumors from chickens infected with MDV (n = 44) or from local lesions induced by inoculation of allogeneic, MDV-infected chick kidney cells (n = 56). Lymphocytes were harvested from these lesions between 4 and 16 days postinoculation and cultured in vitro to establish cell lines. All cell lines expressed Ia antigen and CD3 and/or TCR and thus are activated T cells. Most of the cell lines developed from tumors were CD4+ CD8-; only one cell line was negative for both markers. Sixteen percent of the cell lines were TCR3+, while the remainder were TCR2+. The cell lines developed from local lesions were much more heterogeneous: 45% were CD4- CD8+, 34% were CD4- CD8-, and only 21% were CD4+ CD8-. The number of TCR3+ cell lines was larger than expected for the CD4- CD8+ and CD4- CD8- cell lines, as judged from the presence of these cells in the blood. These results indicate that several subsets of T lymphocytes can be transformed by MDV, depending on the pathogenesis of infection. Activation of T cells as a consequence of the normal pathogenesis or by allogeneic stimulation seem to be a first important step in the process of transformation.

Evolutionary conservation of antigen recognition: the chicken T-cell receptor beta chain

T cells play important regulatory roles in the immune responses of vertebrates. Antigen-specific T-cell activation involves T-cell receptor (TCR) recognition of a peptide antigen presented by a major histocompatibility complex molecule, and much has been learned about this antigen-recognition process through structural and genetic studies of mammalian TCRs. Although previous studies have demonstrated that avian T cells express cell-surface molecules analogous to the mammalian TCR heterodimers, TCR genes have not been identified in nonmammalian species. We now report the cloning of a cDNA that encodes the beta chain of the chicken TCR. Southern blot analysis using this TCR beta cDNA probe demonstrated that the chicken TCR beta locus was clonally rear-ranged in chicken T-cell lines. TCR beta mRNA was expressed in cells isolated from the thymus but not in cells from the bursa of Fabricius where B cells are generated. Sequence analysis of six additional TCR beta cDNAs suggested the existence of at least two variable (V) region families, three joining (J) elements, and single diversity (D) and constant (C) elements. As in mammals, considerable nucleotide diversity was observed at the junctions of the variable, diversity, and joining elements in chicken TCR beta cDNAs. Genomic V beta and J beta elements were also cloned and sequenced. Both elements are flanked by classical heptamer/nonamer recombination signal sequences. Although the chicken and mammalian TCR beta chains displayed only 31% overall amino acid sequence identity, a number of conserved structural features were observed. These data indicate that (i) the chicken TCR beta repertoire is generated by combinatorial and junctional diversity and (ii) despite divergent evolution at the level of nucleotide sequence, important structural features of the TCR beta polypeptide are conserved between avian and mammalian species.

Natural killer cell activity of chicken intraepithelial leukocytes against rotavirus-infected target cells

Intraepithelial leukocytes (IEL) and splenocytes collected from uninfected and rotavirus-infected chickens were evaluated for cytotoxic activity against a natural killer (NK) cell-susceptible lymphoblastoid cell line (LSCC-RP9) and against rotavirus-infected chick kidney cells in 4-h chromium-release assays. Both splenocytes and IELs from uninfected and rotavirus-infected chickens were cytotoxic for LSCC-RP9, and the levels of this NK cell activity were not altered by infection of the host with rotavirus. IELs but not splenocytes from uninfected and rotavirus-infected chickens were cytotoxic for rotavirus-infected but not for uninfected chick kidney cell targets. Because this cytotoxic activity was not induced nor altered by rotavirus infection of the host, and was not major histocompatibility complex-restricted, it was considered to be due to NK cell activity. The cytotoxicity of IELs against rotavirus-infected target cells was dose-dependent; however, there was some suppression of cytotoxic activity at high effector to target cell ratios. There were no differences in the cytotoxic activities of IELs collected from the duodenum versus the jejunum. The in vitro cytotoxic activity of IELs against rotavirus-infected target cells suggested that NK cell activity may be an important immune response to rotavirus infections in vivo. The absence of cytotoxic activity by splenocytes against rotavirus-infected target cells indicated that there may be different subpopulations of NK cells in the spleen and intestinal epithelium of chickens.

Partial transcription map of Marek's disease herpesvirus in lytically infected cells and lymphoblastoid cell lines

Marek's disease herpesvirus (MDV) can cause either a productive-restrictive or lytic infection, a latent infection or can transform thymus-derived lymphocytes. RNA was extracted from infected chicken embryo fibroblasts (CEF) or from lymphoblastoid tumour cell lines. Some of the infected CEF were treated with 200 micrograms/ml cycloheximide to identify immediate early (IE) transcripts, and others with 1 microM 1-(2-fluoro-2-deoxy-B-D-arabinofuranosyl)-5-methyluracil (FMAU), an inhibitor of herpesvirus DNA synthesis to identify early transcripts. An extensive Northern blot analysis was carried out using DNA probes spanning almost the complete MDV genome. In the lytically infected CEF at least 66 discrete transcripts were detected, ranging in size from 9.1 kb to 0.6 kb. Eleven IE transcripts were identified, of which 8 were mapped in the genome segment consisting of the IRL, IRS, US and TRS. Six transcripts were identified as early genes. In the MD lymphoblastoid cell lines MDCC-HPI, a non-producer cell line, and MDCC-CU41, a non-expression cell line, 4 and 7 transcripts were detected, respectively. These RNAs were transcribed from IE genes located mainly in the repeat sequences flanking UL and US and in US. Treatment of the lymphoblastoid cell lines with 20 micrograms/ml 5-iodo-2-deoxyuridine resulted in the additional transcription of 1 RNA species in HPI and 9 in CU41. Most of the transcripts present in lytically infected cells were also detected in MDCC-CU36, a cell line with a high percentage of antigen-positive cells (expression cell line).

Cytotoxic T lymphocytes in reticuloendotheliosis virus-infected chickens

Cytotoxic T lymphocytes were functionally demonstrated in spleen cells from chickens 7 days post inoculation with reticuloendotheliosis virus using a Cr-release assay. Major histocompatibility complex (MHC)-restricted cytotoxicity was demonstrated using effector and target cells from two different strains of chickens of known avian MHC haplotype. Anti-viral specificity was shown and in vivo generation of MHC-restricted cytotoxicity was evaluated. Cytotoxic T cells were distinguished from macrophages and natural killer cells. Their cytotoxicity was not antibody dependent. Higher levels of cytolysis were found with cytotoxic T cells from embryonally bursectomized vs. intact chickens over a large range of effector to target cell ratios. Using monoclonal antibodies, cytotoxic T cells were further defined as Ia+ T cells by immunofluorescence, antibody plus complement-mediated lysis of effector cells and blocking of cytolysis in the Cr-release assay.

Immune response versus susceptibility to Marek's disease

It was hypothesized that the generation of activated T cells through an efficient and rapid immune response during the early pathogenesis of Marek's disease virus (MDV) infection provides a large pool of target cells for transformation. Therefore, the correlation between genetic susceptibility to Marek's disease (MD) and in vitro mitogenic responses of lymphocytes as a measure of cell-mediated immune competence and efficiency was tested. In one series of trials, spleen cells from strains of chickens with differing levels of susceptibility to MD tumors were stimulated with graded doses of Concanavalin A (Con A) or phytohemagglutin (PHA). In a second series of trials, peripheral blood lymphocytes from individual chickens within genetic strains were tested at the same time chickens were challenged with MDV to determine susceptibility. Responsiveness was determined using one-way mixed lymphocyte reaction (MLR) tests as well as mitogen stimulation. Data from the tests comparing chicken strains supported the hypothesis in some but not all cases. The S13 chickens, which are more susceptible than P2a chickens to MD, were significantly more responsive, and highly resistant N2a chickens were significantly less responsive to Con A. In contrast, five other resistant strains were either more responsive (UCD-058, OS13) or equally responsive (UCD-140, OS5, C) to Con A when compared with P2a chickens. The PHA responses were even less predictive of MD susceptibility. No general correlation was observed between responsiveness to either mitogen or MLR tests and subsequent tumor development in trials comparing individuals within strains.