Vaccines | Free Full-Text | How to Break through the Bottlenecks of in Ovo Vaccination in Poultry Farming

It has been fully proven that mature T lymphocytes and B lymphocytes are the primary effector cells of adaptive immune response in chicken. As shown in Figure 2, the thymus gland, the initially formed immune organ in chicken embryos, commences on the 3rd day of incubation and becomes fully mature by the 12th day [8,32,33]. In contrast, the developmental period of bursa of Fabricius, the avian specific immune organ, is much longer, characterized by the gradual establishment of the lymphoid tissue framework, reticulum, from day 4 to day 18 of incubation [32,34]. Along with the maturation of lymphoid organs, the differentiation of lymphocytes takes place. The presence of T and B lymphocytes can be detected for the first time on approximately the 10th day of incubation [33,35,36]. By day 14 of incubation, both the thymus and the bursa of Fabricius, within which T lymphocytes and B lymphocytes develop and mature, respectively, have formed [37]. There are three developmental peaks of T lymphocytes in chicken embryos, namely day 7, day 12 and day 18 of incubation, each lasting 1–3 days [38,39,40]. The enzyme responsible for the reconstitution of T cell antigen receptors (TCRs), known as terminal deoxyribonucleotidyl transferase (TdT), initiates its expression in the thymus on the 12th day of chicken embryo development and exhibits a linear increase during the second developmental peak of T lymphocytes [41,42]. Correspondingly, the recombination of thymic T cell antigen receptor TCRβ occurs at this stage, suggesting the maturation of T lymphocyte immune function [43,44]. The negative selection of autoreactive thymocytes is achieved before day 14 of incubation, and thereafter, the efficacy of in ovo vaccination is not compromised by immunological tolerance [45,46]. From day 8 to day 15 of chicken embryo hatching, B lymphocytes also colonize in the bursa of Fabricius [33,36,47,48,49]. But later than T lymphocytes, the recombination of B cell antigen receptors (BCRs), based on corresponding pairs of functional variable region heavy and light chain genes, occurs from day 15 to day 17 of chicken embryo hatching [48,50]. Therefore, when immunization is carried out on the 14th day of hatching, it is theoretically possible for T lymphocytes to respond to the stimulations of antigen-presenting cells such as dendritic cells and monocytes, so that the activated T lymphocytes further activate B lymphocytes and cytotoxic T lymphocytes to complete the initiation of adaptive immune response in the chicken embryos before hatching out from the shells. However, several former studies have shown that it is until approximately day 18 of hatching that in ovo vaccination is able to effectively initiate the adaptive immune responses in chicken embryos [51,52]. So, what are the differences in the immune cell composition and their immune response processes between the 14-day-old chicken embryos and 18-day-old ones? This becomes the key point to understand the initiation mechanism of adaptive immune response in chicken embryos.

In order to investigate the variations in cellular composition and immune response process during this interval, researchers have conducted some preliminary explorations on the minute structures of thymus and bursa of Fabricius, lymphocyte composition and molecular expression profiles, etc., via histology, embryology, molecular cell biology and multi-omics [53,54,55,56,57,58]. In 2015, Gimeno et al. found that in ovo immunization with MD live vaccine not only triggered T cell activation but also promoted the maturation of immune system in chicken embryos; however, due to the limitations of the research methods and analytical tools of the day, their study lacked further investigation of the patterns of immune cell change following immune system activation [59]. In addition, there are significant differences in the temporal dynamics of adaptive immune responses induced by different types of in ovo vaccines. For instance, effective immune protection can be triggered 6 days post vaccination with live vaccines on the 18th day of chicken embryo hatching, whereas the activation of adaptive immune response by inactivated vaccines requires more than 10 days [31,52]. Identifying the factors contributing to the discrepancy of immune response will also contribute to the comprehension of initiation mechanism of adaptive immune response in chicken embryos.