A quantitative duplex PCR technique for measuring amounts of cell-associated Marek's disease virus: differences in two populations of lymphoma cells

Marek's disease herpesvirus serotype 1 in broiler breeder and layer chickens in Malaysia

Aim:

This study aimed to investigate the occurrence of Marek’s disease (MD) in five poultry farms in Malaysia using postmortem examination, histopathology, and polymerase chain reaction (PCR).

Materials and Methods:

Tissue samples were collected from 24 broiler breeder chickens from four commercial broiler breeder farms and six layer chickens from one layer farm. Gross and histopathological examinations and PCR amplification of the gene encoding for avian MD herpesvirus (MDV-1) were conducted.

Results:

Gross pathological changes including hepatomegaly, splenomegaly, lymphomatous lesion at the mesentery, oviduct atrophy, and follicular atresia with lymphomatous were observed, whereas diffuse multifocal whitish infiltration of the spleen, neoplastic infiltration in the liver, intrafollicular lymphoid infiltration of the bursa of Fabricius, and lymphomatous tumor at the mesentery were seen on histopathological examinations. Confirmation by PCR showed that a total of 16 (53.33%) samples were positive for avian MDV-1. Although the outbreak involved a much larger number of birds in the respective farms, our investigation was limited based on resource and time frame allocated for the study.

Conclusion:

The findings from this study help in emphasizing the potential threats of MDV to the poultry industry globally, in general, and in Malaysia, in particular. As the scope of the current study is limited, future studies focusing on MDV pathogenesis, typing, and causes of vaccine failures are recommended.

Development of reliable techniques for the differential diagnosis of avian tumour viruses by immunohistochemistry and polymerase chain reaction from formalin-fixed paraffin-embedded tissue sections

A variety of techniques have been developed as diagnostic tools for the differential diagnosis of tumours produced by Marek's disease virus from those induced by avian leukosis virus and reticuloendotheliosis virus. However, most current techniques are unreliable when used in formalin-fixed paraffin-embedded (FFPE) tissues, which often is the only sample type available for definitive diagnosis. A collection of tumours was generated by the inoculation of different strains of Marek's disease virus, reticuloendotheliosis virus or avian leukosis virus singularly or in combination. FFPE tissue sections from tumour and non-tumour tissues were analysed by optimized immunohistochemistry (IHC) techniques and traditional as well as quantitative polymerase chain reaction (PCR) with newly designed primers ideal for DNA fragmented by fixation. IHC and PCR results were highly sensitive and specific in tissues from single-infected birds. Virus quantity was higher in tumours compared to non-tumour spleens from Marek's disease (MD) virus-infected birds. Thus, using FFPE sections alone may be sufficient for the diagnosis of MD by demonstration of high quantities of viral antigens or genome in tumour cells, along with the absence of other tumour viruses by traditional PCR, and if standard criteria are met based on clinical history and histology. IHC furthermore allowed detection of the specific cells that were infected with different viruses in tumours from birds that had been inoculated simultaneously with multiple viruses. Following validation with field samples, these new protocols can be applied for both diagnostic and research purposes to help accurately identify avian tumour viruses in routine FFPE tissue sections.

Morphology of immune organs after very virulent plus strain of Marek's disease virus infection in vaccinated hens

Marek's disease (MD) outbreaks in poultry flocks may be associated with overriding of vaccine immune protection by very virulent (vvMDV) or very virulent plus (vv+MDV) strains. This paper presents the study on lymphoid organ morphology in the latent phase of MD caused by vv+MDV which break post-vaccinal protection in hens. We also immunohistochemically examined B and T populations as well as B/T and CD4+/CD8+ ratio of lymphocytes in lymphatic organs and, as a background, in MD lymphomas from non-lymphatic organs. The number of antigen expressed cells was evaluated as a percentage of positive cells in the one power field. Organ samples were collected from 24 dead reproductive hens (Ross 308 line) in age between 35-56 weeks, infected with vv+MDV. The hens originated from farms with MD outbreaks, despite earlier routine vaccination with CVI988/Rispens + HVT. The control organ samples originated from 15 clinically healthy hens at the same age and line, subjected to the same vaccination schedule. The number of CD3+, CD8+ and TCRγδ+ cells was significantly lower in MDV infected thymus, spleen and cecal tonsils in comparison to that found in the control organs. The proportion of CD4+ was also distinctly reduced in the thymus and limited in the spleen of MDV infected hens. This study revealed that infection with field vv+MDV isolates might break post-vaccinal protection and influence the central and peripheral immune system. The decrease in CD8+ and TCRγδ+ cell number in the thymus, spleen and cecal tonsils suggests that primarily these cells are involved in cell-mediated cytotoxicity against MDV transformed cells during latency.

Absolute quantification of a very virulent Marek's disease virus dynamic quantity and distributions in different tissues

Chickens infected with Marek's disease virus (MDV) carry the virus consistently for a long time, which increases the incidence and rate of virus-induced multi-organ tumors and increases its potential for horizontal transmission. There is a positive correlation between very virulent (vv) MDV quantity and the pathology. The purpose of this study was to determine the vvMDV loads dynamics in different phases, and the correlation between the viral quantity and tumor development. We used a SYBR Green duplex real-time quantitative PCR (q-PCR) assay to detect and quantify MDV loads and distributions in different tissues, targeting the Eco-Q protein gene (meq) of the virus and the house-keeping ovotransferrin (ovo) gene of chickens. The q-PCR was performed using different tissue DNA preparations derived from chickens which were infected with 1,000 pfu of the SDWJ1302 strain and tissue samples were collected from control and MDV-infected birds on 7, 10, 15, 21, 28, 40, 60, and 90 d post-infection (DPI). The data indicated that the MDV genome was almost quantifiable in immune organs of infected chickens as early as 7 DPI, and the number of MDV genome copies in the blood and different organs peaked by 28 DPI, but then gradually decreased by 40 DPI. The levels of viral quantity in the lymphocytes, liver, and spleen were all higher than those in other organs, and that in the feather follicles was the highest among different phases of MDV infection. The vvMDV could still be detected in peripheral blood and tissues by 90 DPI, and the vast existence of virus will stimulate tissue destruction. The data provided further evidence of viral infection involving multi-organ distribution and mainly involving immune organ proliferation, resulting in immunosuppression.

Innovation and discovery: the application of nucleic acid-based technology to avian virus detection and characterization

Polymerase chain reaction (PCR)-based approaches to the detection, differentiation and characterization of avian pathogens continue to be developed and refined. The PCRs, or reverse transcriptase-PCRs, may be general, designed to detect all or most variants of a pathogen, or to be serotype, genotype or pathotype specific. Progress is being made with respect to making nucleic acid approaches more suitable for use in diagnostic laboratories. Robotic workstations are now available for extraction of nucleic acid from many samples in a short time, for routine diagnosis. Following general PCR, the DNA products are commonly analyzed by restriction endonuclease mapping (restriction fragment length polymorphism), using a small number of restriction endonucleases, based on a large body of sequence data. Increasingly, however, nucleotide sequencing is being used to analyze the DNA product, in part due to the expanding use of non-radioactive sequencing methods that are safe and enable high throughout. In this review, I highlight some recent developments with many avian viruses: Newcastle disease virus; circoviruses in canary and pigeon; infectious bursal disease virus (Gumboro disease virus); avian adenoviruses, including Angara disease/infectious hydropericardium virus, haemorrhagic enteritis virus of turkeys, and egg drop syndrome virus; avian herpesviruses, including infectious laryngotracheitis virus, duck plague virus, psittacine herpesvirus (Pacheco's parrot disease virus), Marek's disease virus and herpesvirus of turkeys; avian leukosis virus (associated with lymphoid leukosis or myeloid leukosis, and egg transmission); avian pneumoviruses (turkey rhinotracheitis virus); avian coronaviruses, including infectious bronchitis virus, turkey coronavirus and pheasant coronavirus; astrovirus, in the context of poult enteritis and mortality syndrome, and avian nephritis virus; and avian encephalomyelitis virus, a picornavirus related to hepatitis A virus.

Nuclear Factor kappa B is central to Marek's disease herpesvirus induced neoplastic transformation of CD30 expressing lymphocytes in-vivo

Background

Marek’s Disease (MD) is a hyperproliferative, lymphomatous, neoplastic disease of chickens caused by the oncogenic Gallid herpesvirus type 2 (GaHV-2; MDV). Like several human lymphomas the neoplastic MD lymphoma cells overexpress the CD30 antigen (CD30^hi) and are in minority, while the non-neoplastic cells (CD30^lo) form the majority of population. MD is a unique natural in-vivo model of human CD30^hi lymphomas with both natural CD30^hi lymphomagenesis and spontaneous regression. The exact mechanism of neoplastic transformation from CD30^lo expressing phenotype to CD30^hi expressing neoplastic phenotype is unknown. Here, using microarray, proteomics and Systems Biology modeling; we compare the global gene expression of CD30^lo and CD30^hi cells to identify key pathways of neoplastic transformation. We propose and test a specific mechanism of neoplastic transformation, and genetic resistance, involving the MDV oncogene Meq, host gene products of the Nuclear Factor Kappa B (NF-κB) family and CD30; we also identify a novel Meq protein interactome.

Results

Our results show that a) CD30^lo lymphocytes are pre-neoplastic precursors and not merely reactive lymphocytes; b) multiple transformation mechanisms exist and are potentially controlled by Meq; c) Meq can drive a feed-forward cycle that induces CD30 transcription, increases CD30 signaling which activates NF-κB, and, in turn, increases Meq transcription; d) Meq transcriptional repression or activation of the CD30 promoter generally correlates with polymorphisms in the CD30 promoter distinguishing MD-lymphoma resistant and susceptible chicken genotypes e) MDV oncoprotein Meq interacts with proteins involved in physiological processes central to lymphomagenesis.

Conclusions

In the context of the MD lymphoma microenvironment (and potentially in other CD30^hi lymphomas as well), our results show that the neoplastic transformation is a continuum and the non-neoplastic cells are actually pre-neoplastic precursor cells and not merely immune bystanders. We also show that NF-κB is a central player in MDV induced neoplastic transformation of CD30-expressing lymphocytes in vivo. Our results provide insights into molecular mechanisms of neoplastic transformation in MD specifically and also herpesvirus induced lymphoma in general.

Modelling Marek's disease virus (MDV) infection: parameter estimates for mortality rate and infectiousness

Background

Marek's disease virus (MDV) is an economically important oncogenic herpesvirus of poultry. Since the 1960s, increasingly virulent strains have caused continued poultry industry production losses worldwide. To understand the mechanisms of this virulence evolution and to evaluate the epidemiological consequences of putative control strategies, it is imperative to understand how virulence is defined and how this correlates with host mortality and infectiousness during MDV infection. We present a mathematical approach to quantify key epidemiological parameters. Host lifespan, virus latent periods and host viral shedding rates were estimated for unvaccinated and vaccinated birds, infected with one of three MDV strains. The strains had previously been pathotyped to assign virulence scores according to pathogenicity of strains in hosts.

Results

Our analyses show that strains of higher virulence have a higher viral shedding rate, and more rapidly kill hosts. Vaccination enhances host life expectancy but does not significantly reduce the shedding rate of the virus. While the primary latent period of the virus does not vary with challenge strain nor vaccine treatment of host, the time until the maximum viral shedding rate is increased with vaccination.

Conclusions

Our approach provides the tools necessary for a formal analysis of the evolution of virulence in MDV, and potentially simpler and cheaper approaches to comparing the virulence of MDV strains.

The neoplastically transformed (CD30hi) Marek's disease lymphoma cell phenotype most closely resembles T-regulatory cells

INTRODUCTION

Marek's disease (MD), a herpesvirus-induced lymphoma of chickens is a unique natural model of CD30-overexpressing (CD30hi) lymphoma. We have previously proposed that the CD30hi neoplastically transformed CD4+ T cells in MD lymphomas have a phenotype antagonistic to cell mediated immunity. Here were test the hypothesis that the CD30hi neoplastically transformed MD lymphoma cells have a phenotype more closely resembling T-helper (Th)-2 or regulatory T (T-reg) cells.

MATERIALS AND METHODS

We separated ex vivo-derived CD30hi, from the CD30lo/- (non-transformed), MD lymphoma cells and then quantified the relative amounts of mRNA and proteins for cytokines and other genes that define CD4+ Th-1, Th-2 or T-reg phenotypes.

RESULTS AND DISCUSSION

Gene Ontology-based modeling of our data shows that the CD30hi MD lymphoma cells having a phenotype more similar to T-reg. Sequences that could be bound by the MD virus putative oncoprotein Meq in each of these genes' promoters suggests that the MD herpesvirus may play a direct role in maintaining this T-reg-like phenotype.

Relationship between Marek's disease virus load in peripheral blood lymphocytes at various stages of infection and clinical Marek's disease in broiler chickens

Vaccination with herpesvirus of turkey (HVT) vaccine provides protection against clinical Marek's disease (MD) but does not preclude infection with wild-type MD virus (MDV). The quantity of MDV detected in circulating lymphocytes during the early period after infection may be a useful predictor of subsequent clinical MD later in the life. A study was designed to quantify MDV and HVT copy number in peripheral blood lymphocytes (PBL) using real-time polymerase chain reaction between days 5 and 35 post-challenge and to relate this to subsequent development of gross MD lesions. Female commercial broiler chickens were vaccinated with HVT or were sham-vaccinated at hatch, then challenged with MDV strain MPF-57 at day 2 post-vaccination and reared in positive-pressure isolators up to 56 days post-challenge, when all survivors were euthanized. All dead and euthanized chickens were examined post mortem for gross MD lesions. Birds were scored for MD lesions and mortality. MDV and HVT genome copy numbers were determined for each PBL sample. There was an increase in HVT load in PBL between days 7 and 37 post-vaccination, with marked increases between days 7 and 16 and again between days 30 and 37. There was a steady increase in MDV load to 35 days post-challenge. The mean MDV copy number (log(10)) was greater in chickens subsequently exhibiting gross MD lesions (5.05 +/- 0.21) than in those that did not (2.88 +/- 0.223), with the largest difference at 14 and 21 days post-challenge (P < 0.001). Quantification of MDV during early infection is therefore a potential tool for monitoring MD in broiler flocks.

Rapid, reliable and inexpensive quality assessment of biotinylated cRNA

The interpretation of oligonucleotide array experiments depends on the quality of the target cRNA used. cRNA target quality is assessed by quantitative analysis of the representation of 5' and 3' sequences of control genes using commercially available Test arrays. The Test array provides an economically priced means of determining the quality of labeled target prior to analysis on whole genome expression arrays. This manuscript validates the use of a duplex RT-PCR assay as a faster (6 h) and less expensive (<10 US dollars) but equally accurate alternative to the Test arrays in determining biotinylated cRNA quality. Forty-one different cRNA samples were hybridized to HG-U133A microarrays from Affymetrix. Ten cRNA samples with a beta-actin 3'/5' ratio >6 were chosen and classified as degraded cRNAs, and 31 samples with a beta-actin 3'/5' ratio <6 were selected as good quality cRNAs. Blinded samples were then used for the RT-PCR assay. After gel electrophoresis, optical densities of the amplified 3' and 5' fragments of beta-actin were measured and the 3'/5' ratio was calculated. There was a strong correlation (r(2) = 0.6802) between the array and the RT-PCR beta-actin 3'/5' ratios. Moreover, the RT-PCR 3'/5' ratio was significantly different (P < 0.0001) between undegraded (mean +/- SD, 0.34 +/- 0.09) and degraded (1.71 +/- 0.83) samples. None of the other parameters analyzed, such as i) the starting amount of RNA, ii) RNA quality assessed using the Bioanalyzer Chip technology, or iii) the concentration and OD260/OD280 ratio of the purified biotinylated cRNA, correlated with cRNA quality.

Absolute quantitation of Marek's disease virus and Herpesvirus of turkeys in chicken lymphocyte, feather tip and dust samples using real-time PCR

The further development of Taqman quantitative real-time PCR (qPCR) assays for the absolute quantitation of Marek's disease virus serotype 1 (MDV1) and Herpesvirus of turkeys (HVT) viruses is described and the sensitivity and reproducibility of each assay reported. Using plasmid DNA copies, the lower limit of detection was determined to be 5 copies for the MDV1 assay and 75 copies for the HVT assay. Both assays were found to be highly reproducible for Ct values and calculated copy numbers with mean intra- and inter-assay coefficients of variation being less than 5% for Ct and 20% for calculated copy number. The genome copy number of MDV1 and HVT viruses was quantified in PBL and feather tips from experimentally infected chickens, and field poultry dust samples. Parallelism was demonstrated between the plasmid-based standard curves, and standard curves derived from infected spleen material containing both viral and host DNA, allowing the latter to be used for absolute quantification. These methods should prove useful for the reliable differentiation and absolute quantitation of MDV1 and HVT viruses in a wide range of samples.

Classification of Marek's disease viruses according to pathotype: philosophy and methodology

The concept of pathotype in Marek's disease (MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv+ pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable.The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens (15x7 ab+), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A (American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease (both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria (such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit" comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria (see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.

Absolute quantitation of Marek's disease virus genome copy number in chicken feather and lymphocyte samples using real-time PCR

A real-time PCR method was developed, optimised and validated, to enable quantitation of Marek's disease virus genomes as copy number per million host cells. The duplex PCR measured the virus meq gene and host ovotransferrin gene in a single reaction enabling correction for differences in amount of sample DNA added. A bacterial artificial chromosome (BAC) clone of the virus genome, and a plasmid (pGEM-T-ovo) bearing a fragment of the chicken ovotransferrin gene, were used to quantify virus and host genomes respectively. This sensitive and reproducible assay was established initially using chicken lymphocyte DNA, then adapted for feather tip DNA by inclusion of bovine serum albumin in the reaction to overcome inhibition by melanin. The principal advantages are: (1) determination of absolute virus genome copy number enabling meaningful comparison between samples; (2) expression of copy number per million cells, allowing direct correlation with plaque assays; (3) using BAC-cloned whole virus genome as a standard potentially enables any virus gene to be used as the PCR target. This is the first report of quantitation of MDV genomes in feather tips, and application of this assay could significantly further our understanding of pathogenesis, spread, diagnosis, genetic resistance and vaccinal control of Marek's disease.

Interferon-Containing Supernatants Increase Marek's Disease Herpesvirus Genomes and Gene Transcription Levels, but Not Virion Replicationin Vitro

Viruses encounter the innate immune system immediately after infection of the host; specifically, soluble molecules that are both directly lethal and that initiate acquired immunity. Using the oncogenic Marek's disease alpha-herpesvirus (MDV) model, we quantified the effect of a interferon-containing supernatants (ICS), on MDV replication, gene transcription and antigen expression kinetics. We used an established cell culture system and a well-defined virulent MDV (RB-1B). RB-1B was cultured without ICS, or pretreated and then continuously treated with ICS. We compared (i) RB-1B infectivity; (ii) RB-1B growth by microscopy; (iii) numbers of cells expressing RB-1B antigens by flow cytometry; (iv) RB-1B-DNA load per cell by duplex real-time PCR, and (v) gene transcription kinetics for key MDV-life stages by duplex real-time reverse-transcriptase PCR (RT-PCR). ICS inhibited RB-1B infection, completion of productive life cycle and cell-to-cell infection. The numbers of cells expressing glycoprotein B (a kinetically late antigen) greatly decreased, but the numbers of cells expressing pp38 (a kinetically early antigen) decreased only slightly. The two greatest effects were increases in both RB-1B-DNA per infected cell and pp38 mRNA. We propose MDV has evolved to increase specific gene transcription and genome copies per cell to compensate for ICS. We speculate that the bi-directional shared pp38/origin of replication promoter, is central to this mechanism.

Identification of the neoplastically transformed cells in Marek's disease herpesvirus-induced lymphomas: recognition by the monoclonal antibody AV37

Understanding the interactions between herpesviruses and their host cells and also the interactions between neoplastically transformed cells and the host immune system is fundamental to understanding the mechanisms of herpesvirus oncology. However, this has been difficult as no animal models of herpesvirus-induced oncogenesis in the natural host exist in which neoplastically transformed cells are also definitively identified and may be studied in vivo. Marek's disease (MD) herpesvirus (MDV) of poultry, although a recognized natural oncogenic virus causing T-cell lymphomas, is no exception. In this work, we identify for the first time the neoplastically transformed cells in MD as the CD4(+) major histocompatibility complex (MHC) class I(hi), MHC class II(hi), interleukin-2 receptor alpha-chain-positive, CD28(lo/-), phosphoprotein 38-negative (pp38(-)), glycoprotein B-negative (gB(-)), alphabeta T-cell-receptor-positive (TCR(+)) cells which uniquely overexpress a novel host-encoded extracellular antigen that is also expressed by MDV-transformed cell lines and recognized by the monoclonal antibody (MAb) AV37. Normal uninfected leukocytes and MD lymphoma cells were isolated directly ex vivo and examined by flow cytometry with MAb recognizing AV37, known leukocyte antigens, and MDV antigens pp38 and gB. CD28 mRNA was examined by PCR. Cell cycle distribution and in vitro survival were compared for each lymphoma cell population. We demonstrate for the first time that the antigen recognized by AV37 is expressed at very low levels by small minorities of uninfected leukocytes, whereas particular MD lymphoma cells uniquely express extremely high levels of the AV37 antigen; the AV37(hi) MD lymphoma cells fulfill the accepted criteria for neoplastic transformation in vivo (protection from cell death despite hyperproliferation, presence in all MD lymphomas, and not supportive of MDV production); the lymphoma environment is essential for AV37(+) MD lymphoma cell survival; pp38 is an antigen expressed during MDV-productive infection and is not expressed by neoplastically transformed cells in vivo; AV37(+) MD lymphoma cells have the putative immune evasion mechanism of CD28 down-regulation; AV37(hi) peripheral blood leukocytes appear early after MDV infection in both MD-resistant and -susceptible chickens; and analysis of TCR variable beta chain gene family expression suggests that MD lymphomas have polyclonal origins. Identification of the neoplastically transformed cells in MD facilitates a detailed understanding of MD pathogenesis and also improves the utility of MD as a general model for herpesvirus oncology.

Resistance to Marek's disease herpesvirus-induced lymphoma is multiphasic and dependent on host genotype

Genotype-dependent differences in Marek's disease (MD) susceptibility were identified using 14-day-old line N and 6(1) (resistant) and 151 and 7(2) (susceptible) inbred chickens infected with HPRS-16 MD virus (MDV). All line 72 chickens developed progressive MD. Line 15I had fluctuating MD-specific clinical signs and individuals recovered. A novel histologic scoring system enabled indices to be calculated for lymphocyte infiltration into nonlymphoid organs. All genotypes had increased mean lesion scores (MLSs) and mean total lesion scores after MDV infection. These differed quantitatively and qualitatively between the genotypes. Lines 6(1) and 7(2) had a similar MLS distribution in the cytolytic phase, although scores were greater in line 7(2). At the time lymphomas were visible in line 7(2), histologic lesions in line 6(1) were regressing. AV37+ cells were present in similar numbers in all genotypes in the cytolytic phase, suggesting that neoplastically transformed cells were present in all genotypes regardless of MD susceptibility. After the cytolytic phase, AV37+ cell numbers increased in lines 7(2) and 15I but decreased in lines 6(1) and N. In the cytolytic and latent phases, in all genotypes, most infiltrating cells were CD4+. After this time, line 7(2) and 15I lesions increased in size and most cells were CD4+; line 6(1) and N lesions decreased in size and most cells were CD8+. In all genotypes, AV37 immunostaining was weak in lesions with many CD8+ cells, suggesting that AV37 antigen expression or AV37+ cells were controlled by CD8+ cells. The rank order, determined by clinical signs and pathology, for MD susceptibility (highest to lowest) was 7(2) > 15I > 6(1) > N.