Evaluation of factors influencing replication of serotype 1 Marek's disease vaccines in the chicken lung

Replication ability of three highly protective Marek's disease vaccines: implications in lymphoid organ atrophy and protection

The present work is a chronological study of the pathogenesis of three attenuated serotype 1 Marek's disease (MD) virus strains (RM1, CVI988 and 648A80) that provide high protection against MD but have been attenuated by different procedures and induce different degrees of lymphoid organ atrophy. All studied strains replicated in the lymphoid organs (bursa,x thymus and spleen) and a peak of replication was detected at 6 days post inoculation (d.p.i.). Differences, however, were observed among vaccine strains. RM1 strain replicates more in all lymphoid organs compared with CVI988 and 648A80 strains. In addition, replication of RM1 in the thymus did not decrease after 6 d.p.i. but continued at high levels at 14 d.p.i. and until the thymus was completely destroyed. Lung infection occurred very early after infection with all of the three vaccines and the level of replication was similar to that found in the lymphoid organs. Infected cells were very large and appeared scattered in the lung parenchyma and in the parabronchial lining. The study of the target cells for the early infection in cell suspensions of blood and spleen showed that both non-adherent cell populations (enriched in lymphoid cells) and adherent cells (enriched in monocytes/macrophages) supported MD virus infection. Infection in adherent cells was especially high at very early stages of the infection (3 to 6 d.p.i.). Atrophy of lymphoid organs is a major drawback in the production of highly protective vaccines against MD. A better understanding of the mechanisms associated with lymphoid organ atrophy will aid in overcoming this problem.

Cost effective and time efficient measurement of CD4, CD8, major histocompatibility complex Class II, and macrophage antigen expression in the lungs of chickens

Cells expressing CD4, CD8, major histocompatibility complex (MHC) Class II, and macrophage biomarkers in lungs of chickens were quantified by measuring total area of antigen expressed using imageJ, a software program developed at the National Institutes of Health and available at no cost. The procedures reported here were rapid, and reproducible. Total area of antigen expressed had positive correlation with manual counts of cells expressing CD4 and CD8 biomarkers after inoculation with serotype 1 Marek's disease virus (MDV) vaccines. Visual inspection and overlays prepared from outlines of cells counted by imageJ confirmed agreement between antigen expression and area measured. Total area measured was not dependent on time of image acquisition from randomly selected fields from the same slides. Total area values were not computer specific, but acquisition of the original images required standardization of microscope used and camera setup. All steps in the process from sample collection through sectioning, staining, and image acquisition must be standardized as much as possible. Chickens infected with a very virulent+ (vv(+)) isolate of MDV (648A) had increased CD4, CD8, MHC Class II, and macrophage biomarker expression compared to noninfected control chickens at 10 days post infection, but variable responses depending on the specific biomarker measured at 3 and 5 days post infection. The procedure described here is faster and more reproducible than manual counting in cases (CD4 and CD8) where the number of positive cells is low enough for manual counts. Manual counting is not possible with MHC Class II and macrophage antigens nor when CD4(+) cells are present in large numbers following proliferation to tumors, thus subjective systems are used for scoring in these conditions. Using imageJ as described eliminates the need for subjective and less reproducible methods for measuring expression of these antigens.

Mortality of one-week-old chickens during naturally occurring Marek's disease virus infection

Marek's disease (MD) is a disease of chickens that occurs worldwide and has serious economic consequences. MD can present as one of several forms, with the most commonly occurring forms being the lymphoproliferative diseases. Under experimental conditions, an early mortality syndrome has been recognized following infection by some but not all strains of MD virus (MDV). This is the first report of a confirmed case of mortality due to naturally occurring MDV infection in 1-week-old, nonvaccinated, chickens. Necrotizing lesions were observed in the bursa of Fabricius, lung, duodenum, jejunum, and proventriculus, and large intranuclear inclusion bodies were a striking feature in tissues with lesions in all birds. Immunohistochemical staining for the pp38 protein of MDV revealed abundant pp38 antigen in the affected tissues, confirming the presence of MDV within the lesions. PCR yielded an amplicon with 97% homology to the meq gene of MDV. No evidence of co-infection by either of the immunosuppressive agents chicken anemia virus and infectious bursal disease virus was detected.