Veterinary Science Collected Works - Theses

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End date: 2022 × Type: PhD thesis × Subject: Antibody response × Subject: Infectious laryngotracheitis virus ×

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    Chicken immune responses to infectious laryngotracheitis virus (ILTV) glycoproteins
    Sabir, Ahmad Jawad ( 2019)
    Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes acute respiratory disease, primarily infecting the upper respiratory tract and conjunctiva. The disease results in significant economic losses to the poultry industry worldwide. Currently, the immune status of the vaccinated flocks or individuals is assessed by the presence of systemic antibodies using commercially available whole virus ELISAs. These ELISAs are good indicators of exposure to either field or vaccine virus, however, the assays are poor in predictors of the level of protective immunity. In this study, individual ILTV glycoproteins (gC, gD, gE, gG, gI and gJ) were assessed for their potential to predict the levels of protective antibody and/or cell-mediated immunity as well as for their capacity to induce neutralising antibodies. To examine whether antibody responses to individual ILTV glycoproteins are correlated with disease and protection, individual glycoproteins expressed in mammalian cells were used in indirect ELISAs to measure post-vaccination and/or post-challenge chicken serum antibodies. Serum optical density (OD) values detected by the whole virus, gC, gI and gJ were significantly higher in birds vaccinated with the Serva vaccine strain compared to the SA2 vaccine strain. However, the mean ODs detected by gD, gE and gG were not significantly different between the vaccine strains. Examination of post-vaccination pre-challenge antibodies to individual glycoproteins did not find a strong correlation between systemic antibodies to individual glycoproteins and protection. The possibility that the protective epitopes of glycoproteins may not be readily detectable in ELISA was investigated by evaluation of the antibodies to each glycoprotein in an in-vitro virus neutralisation assay. Monospecific polyclonal antibodies to individual ILTV glycoproteins gC, gD, gE, gI and gJ were generated in rats and examined for their capacity to neutralise the virus. Neutralising antibodies were detected to gC, gD and gJ individually or in combination. Polyclonal antibodies to gE and gI failed to neutralise the virus when tested individually or combined. In-vitro splenocyte stimulation assays using individual glycoprotein stimulating antigen were conducted to examine the transcription profiles of selected cytokines from nonvaccinated-nonchallenged (NVXNCh), vaccinated-challenged (VXCh) and nonvaccinated-challenged (NVXCh) groups of chickens. The transcription profiles of cytokines referencing Th1, Th2 and Th17 responses were then examined against tracheal pathology results to identify correlation between responses against viral glycoprotein(s) and protective immune response. Expression of IFN-gamma was significantly upregulated in VXCh and NVXCh groups of birds in response to stimulation with gD, with expression levels moderately correlated with protection. Stimulation with gE and gI resulted in significantly higher levels of IL-6 in VXCh birds and response to gE was moderately correlated with protection. Expression of IL-17a was significantly elevated in response to stimulation with gD in NVXCh birds and results were moderately linked to disease severity. The results of this study suggest that IFN-gamma and IL-6 cytokine responses to gD and gE, respectively, may be correlated with protective cell-mediated immunity, whereas IL-17a cytokine responses to gD may be linked to disease. Finally, whole-genome analysis of an unusually virulent “vaccine-like” isolate revealed a new class of ILTV, identified here as class 7b, emerged as a result of recombination probably between another recombinant strain and a vaccine strain. Further analysis of the genome detected recombination hotspots within the unique long (UL) region comprising of genes encoding gB, gC and gM and assembly proteins UL28 and ICP18.5. Boot-scanning analysis of concatenated glycoprotein sequence alignments detected recombination breakpoints within glycoproteins C and H, but not within glycoproteins located in the unique short (US) region. Further studies would be needed to explore the possible role of these glycoproteins in virulence. Thus, the studies reported in this thesis have contributed to understanding (1) the relationship between antibodies to ILTV glycoproteins and protective immunity, (2) virus neutralisation potential of specific antibodies to ILTV glycoproteins, (3) the potential role of ILTV glycoproteins to induce cell-mediated protective immunity, (4) and laid foundation to explore inter-strain genetic diversity within ILTV glycoproteins.