The Immunological Capacity of Thrombocytes

Abstract

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.

Keywords: adaptive immune responses; antigen-presenting cell; innate immune response; thrombocyte.

Figure 1

Comparison of immune molecules reported to be present in/on avian (left) and ichthyoid (right) thrombocytes, which lend support to the unique role of this cell type in innate and adaptive immunity. The figure is divided into left (avian) and right (ichthyoid) sides with several receptors and molecules associated with antigen presentation on the cell surface and in the cytoplasm of thrombocytes. The markers in both bird and fish thrombocytes have been compiled from several refereed publications examining the immune characteristics of this cell type. TLR: toll-like receptor; CD: cluster of differentiation; MHC: major histocompatibility complex; TREM: Triggering Receptors Expressed on Myeloid cells; CLEC: C-type lectin-like receptors; SLAM: signaling lymphocytic activating molecule; CCR: C-C chemokine receptor; CXCR: C-X-C receptor; c-Mpl: thrombopoietin receptor. Created with BioRender.com (accessed on 26 July 2023).

Figure 2

Genes of the TLR signaling pathway observed in chicken thrombocytes. TLR: toll-like receptor; MD2: myeloid differentiation protein 2; CD14: cluster of differentiation 14; LPS: lipopolysaccharide; MyD88: myeloid differentiation primary-response protein 88; TIRAP: Toll/Il-1 receptor domain-containing adaptor protein; TRIF: Toll/Il-1 receptor domain-containing adaptor inducing IFN-β; IRAK: IL-1 receptor-associated protein kinase; TRAF: tumor necrosis factor receptor-associated factor; NF-κB-nuclear factor κB; TANK-TRAF-family-member-associated NF-κB activator; MAPKs: mitogen-activated protein kinases; AP-1: activator protein; IRF: interferon regulatory factor; IKK-IκB kinase; and ERK: extracellular signal-regulated kinase. Created with BioRender.com (Accessed on 26 July 2023).

Figure 3

Constitutively expressed genes of class I and II major histocompatibility complex processes of endogenous and exogenous antigen processing pathways found in chicken thrombocytes. B2M: β2-microglobulin; BF2: MCH I alpha chain 2; BLB1: MHC II beta chain; CALR: calrecticulin; CANX: calnexin; CD74: invariant chain of CLIP; DMA: MHC II DM alpha chain; TAPBP: tapasin; TAP1/2: transporter 1/2. Created with BioRender.com (Accessed on 27 July 2023).