Adaptive immunity and antigen processing in fish

Aquaculture suffers from infectious illnesses. Fish are the first animal group to include the majority of the fundamental components of the adaptive immune system. Nonetheless, given the vast variety of fish, many basic distinctions occur across species. Similarly, the fish adaptive immune system has certain distinct characteristics. These anatomical and functional distinctions between fish and mammals foreshadow significant variations in how fish manage both humoral and cellular adaptive immune responses. Adaptive immunity is distinguished by two key characteristics: specific antigen recognition and the formation of immunological memory. Thus, B and T cells combine to build an adaptive immune system that recognizes and combats infection and illness with precision. B- and T-cell progenitors reorganize their particular receptors (BCR and TCR, respectively), resulting in a wide spectrum of antigen recognition domains. The BCR and its secreted form (antibodies) identify and activate humoral immune responses to antigens in their original state. TCRs, on the other hand, bind to antigens processed and displayed on major histocompatibility complex (MHC) molecules, resulting in cellular immune responses. Subsequent contacts with the antigen elicit more effective and quicker immune responses through the formation of both B and T memory cells, forming the foundation for vaccination. The adaptive immune response relies heavily on antigen processing and presentation (APP) to immune effector cells that combat infections. The polymorphic genes encoding the major histocompatibility complex are at the heart of the adaptive immunity that evolved in lower animals (MHC). MHC class I molecules are mostly composed of peptides that have been digested in the cytosol by the proteasome and delivered to the cell surface of all cells through secretory compartments. MHC class II molecules are also expressed by professional antigen-presenting cells (pAPC), which generally deliver peptides processed from external antigens through lysosomal processes. Autophagy is an internal self-degradation mechanism that is common across all eukaryotes and is activated by hunger to help maintain cellular homeostasis. Autophagy is characterized by the fusion of autophagosomes, which engulf sections of the cytosol and merge with lysosomes (macroautophagy), or by chaperones, which bring proteins to lysosomes (chaperone-mediated autophagy, CMA). Thus, antigens may be digested during self-degradation and presented by the MHC to immunological effector cells, tying autophagy to APP.

Keywords:

Adaptive immune system , B- and T-cells , Antigen-presenting cells