Chicken TREM-B1, an Inhibitory Ig-Like Receptor Expressed on Chicken Thrombocytes

Delineation of chicken immune markers in the era of omics and multicolor flow cytometry

Multiparameter flow cytometry is a routine method in immunological studies incorporated in biomedical, veterinary, agricultural, and wildlife research and routinely used in veterinary clinical laboratories. Its use in the diagnostics of poultry diseases is still limited, but due to the continuous expansion of reagents and cost reductions, this may change in the near future. Although the structure and function of the avian immune system show commonalities with mammals, at the molecular level, there is often low homology across species. The cross-reactivity of mammalian immunological reagents is therefore low, but nevertheless, the list of reagents to study chicken immune cells is increasing. Recent improvement in multicolor antibody panels for chicken cells has resulted in more detailed analysis by flow cytometry and has allowed the discovery of novel leukocyte cell subpopulations. In this article, we present an overview of the reagents and guidance needed to perform multicolor flow cytometry using chicken samples and common pitfalls to avoid.

The Immunological Capacity of Thrombocytes

Thrombocytes are numerous in the blood of aves (birds) and ichthyoids (fish). The origin of this cell type is a common hematopoietic stem cell giving rise to a cell that is active in blood coagulation, inflammatory functions, and the immune response in general. It has been well documented that thrombocytes can phagocytize small particles and bacteria. While phagocytosis with an associated oxidative burst has been reported for chicken thrombocytes, some questions remain as to the degradation capacity of phagosomes in ichthyoids. As innate cells, thrombocytes can be stimulated by bacterial, viral, and fungal pathogens to express altered gene expression. Furthermore, there have been observations that led researchers to state that platelets/thrombocytes are capable of serving as "professional antigen presenting cells" expressing CD40, CD80/86, MHC I, and MHC II. This indeed may be the case or, more likely at this time, provide supporting evidence that these cells aid and assist in the role of professional antigen-presenting cells to initiate adaptive immune responses.

CD11d is a novel antigen on chicken leukocytes

In life sciences, antibodies are among the most commonly used tools for identifying, tracking, quantifying and isolating molecules, mainly proteins. However, it has recently become clear that antibodies often fall short with respect to specificity and selectivity and in many cases target proteins are not even known. When commercial availability of antibodies is scarce, e.g. for targeting proteins from farm animals, researchers face additional challenges: they often have to rely on cross-reactive antibodies, which are poorly characterized for their exact target, their actual cross-reactivity and the desired application. In this study, we aimed at identifying the true target of mouse monoclonal antibody 8F2, which was generated against chicken PBMC and used for decades in research, while it's actual target molecule remained unknown. We used 8F2 antibody for immunoprecipitation in chicken PBMC and subsequently identified its true target as CD11d, which was never described in chicken lymphocytes before, by quantitative LC-MSMS. The most abundant interactor of CD11d was identified as integrin beta 2. The existence of this alpha integrin was therefore clearly proven on protein level and provides a first basis to further assess the role of CD11d in chickens in future studies. Data are available via ProteomeXchange with identifier PXD017248. SIGNIFICANCE: Our studies determined CD11d as the true target of a previously uncharacterized mouse monoclonal antibody 8F2, generated against chicken peripheral blood derived mononuclear cells (PBMC). This is therefore now first member of alpha integrins in chickens, that existence was now clearly identified on protein level. The additional identification of CD11d interactors provides information on integrin-dependent regulation of signaling networks, allowing further functional studies.

Splenic γδ T cell subsets can be separated by a novel mab specific for two CD45 isoforms

CD45 isoforms have been identified in a variety of different species and mab against various isoforms have been instrumental to define cellular subsets. In the process of generating novel mab against chicken γδ T cells two mab with specificity for CD45 were identified and characterized. The analysis of the chicken CD45 genomic structure suggested three exons being involved in alternative splicing. We cloned and expressed the full length CD45 isoform and three shorter isoforms. While the 7D12 mab reacted with all of these isoforms, the 8B1 mab selectively reacted with two short isoforms lacking either exons 3 and 5 or exons 3, 5 and 6. As expected, the reactivity of 7D12 included all leukocyte subsets, also including thrombocytes. In contrast, the 8B1 mab only reacted with lymphocytes and monocytes. 8B1 expression was found on almost all blood αβ T cells, while a γδ T cell subset and virtually all B cells lacked 8B1 reactivity. The fraction of 8B1^- αβ and γδ cells was larger in splenocytes as compared to PBL and there was also a population of 8B1⁺ splenic B cells. CD3 stimulation of splenic T cells resulted in upregulation of the 8B1 antigen on all T cells. Three-color immunofluorescence revealed differences in CD28 expression between the 8B1⁺ and 8B1¯ γδ T cell subsets with a higher CD28 expression level on 8B1¯ cells. The CD28 antigen was upregulated upon stimulation of the cells with IL-2 and IL-12. This novel mab will be a useful tool to further analyze chicken γδ T cells in more detail.