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CD4: a vital player in the teleost fish immune system

Ashfaq, Hassan, Soliman, Hatem, Saleh, Mona, El-Matbouli, Mansour
Veterinary research 2019 v.50 no.1 pp. 1
CD4-positive T-lymphocytes, antigens, cytokines, dendritic cells, fish, glycoproteins, humoral immunity, immune response, major histocompatibility complex, pathogens, transcription factors
CD4 is a nonpolymorphic transmembrane glycoprotein molecule that is expressed on the surface of T-helper cells and plays an essential role in the immune response. It functions as a coreceptor with the T-cell receptor by binding to major histocompatibility complex class II on the surface of dendritic cells that present antigens. CD4⁺ T cells hold a key position in coordinating the immune system through production of several cytokines after activation and differentiation. The CD4⁺ T helper subtypes (T-helper 1, T-helper 2, T-helper 17, T-helper 9, and regulatory-T cells) perform different immune functions subsequent to their differentiation from the naive T cells. Different types of CD4⁺ T cells require different cytokines such as drivers and effectors, as well as master transcription factors for their activation. Fish cells that express CD4-related genes are activated in the presence of a pathogen and release cytokines against the pathogen. This review highlights the types of CD4⁺ T cells in fish and describes their direct role in cell-mediated and humoral immunity for protection against the intracellular bacterial as well as viral infections in fish.