Cluster of differentiation antigens: essential roles in the identification of teleost fish T lymphocytes

Abstract

Cluster of differentiation (CD) antigens are cell surface molecules expressed on leukocytes and other cells associated with the immune system. Antibodies that react with CD antigens are known to be one of the most essential tools for identifying leukocyte subpopulations. T lymphocytes, as an important population of leukocytes, play essential roles in the adaptive immune system. Many of the CD antigens expressed on T lymphocytes are used as surface markers for T lymphocyte classification, including CD3, CD4 and CD8 molecules. In this review, we summarize the recent advances in the identification of CD molecules on T lymphocytes in teleosts, with emphasis on the functions of CD markers in the classification of T lymphocyte subsets. We notice that genes encoding CD3, co-receptors CD4 and CD8 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. T lymphocytes can be divided into CD4⁺ and CD8⁺ cells discriminated by the expression of CD4 and CD8 molecules in teleost, which are functionally similar to mammalian helper T cells (Th) and cytotoxic T cells (Tc), respectively. Further studies are still needed on the particular characteristics of teleost T cell repertoires and adaptive responses, and results will facilitate the health management and development of vaccines for fish.

Keywords: Classification; Fish; Monoclonal antibody; Surface marker; T lymphocytes.

Fig. 1

Cluster of differentiation antigens expressed on T lymphocytes in teleost fish. According to the definition and nomenclature of CD antigens in mammals, many of CD molecules expressed on T lymphocytes have also been cloned and identified in multiple fish species. For example, CD3 and TCR molecules coexist on T cells, forming the basic structure of T cell antigen-specific recognition and cell activation signal transmission. T lymphocytes can be divided into T helper (Th) and T cytotoxic (Tc) cells, distinguished by the expression of CD4 and CD8 glycoprotein, respectively. The counter-receptors of CD28 and CTLA4, as important T cell costimulatory receptors, are involved in activation or inactivation of T cells. CD2, CD40L and other cluster differentiation antigens are also characterized in teleost fish

Fig. 2

Development of monoclonal antibodies against CD marker molecules in fish. For the selection of immunogens, the eukaryotic plasmids, purified recombinant proteins or peptide-KLH complexes are used as antigens. After immunization of mouse, splenocytes and myeloma cells are fused and then the positive hybridoma cells are immunologically screened. The positive hybridoma cells were expanded and cultured, and the cell supernatant was collected as monoclonal antibody. The monoclonal antibodies against CD molecules are important tools for the identification of T cell subsets in fish

Fig. 3

Classification of T cell subsets in mammals and teleost fish. In mammals, there is only one CD4 gene, so based on the expression of CD4 and CD8 co-receptors, T cells can be classified into CD4⁺ and CD8⁺ T cell subsets, where CD4⁺ T cells are functionally known as T helper cells (Th) and CD8⁺ T cells are called cytotoxic T cells (Tc or CTL). CD4⁺ and CD8⁺ T cells were also identified in teleost fish. In contrast, two CD4 genes (CD4-1 and CD4-2) were cloned in fish, and CD4⁺ T cells can be classified into CD4-1 single positive (CD4-1 SP), CD4-1 and CD4-2 double positive (CD4DP) and CD4-2 single positive (CD4-2 SP) cells

Fig. 4

Activation and differentiation of CD4⁺ T lymphocytes. After stimulation by different antigens, signal 1 (binding of the T-cell receptor (TCR) to the peptide-MHCII complex on the antigen-presenting cell (APC) surface) and signal 2 (binding of the T-cell co-receptor CD28 to CD80/CD86 on the APC surface) are required for T-cell activation. The binding of polarizing cytokines to their respective receptor on the T cell surface represents signal 3. Different combinations of these cytokines influence T cell differentiation into distinct effector T cell subtypes (Th1, Th2, Th17, and Treg cells) that produce signature cytokines