Cytokines

Cytokines are soluble polypeptide mediators synthesized and released by cells for the purposes of intercellular signaling and communication. Table 1-3 lists examples of cytokines associated with ocular inflammation. Various types of intercellular signaling occur, including paracrine (signaling of neighboring cells at the same site), autocrine (stimulation of a receptor on its own surface, and endocrine (action on a distant site through release into the blood).

Traditionally, investigators have subdivided cytokines into families with related activities, sources, and targets, using terms such as growth factors, interleukins, lymphokines, interferons, monokines, and chemokines. Thus, growth factor traditionally refers to cytokines mediating cell proliferation and differentiation. The terms interleukin and lymphokine identify cytokines thought to mediate intercellular communication among lymphocytes or other leukocytes. Interferons are cytokines that limit or interfere with the ability of a virus to infect a cell. Monokines are immunoregulatory cytokines secreted by monocytes and macrophages. Chemokines are chemotactic cytokines. Although some cytokines are specific for particular cell types, most have such degrees of multiplicity and redundancy of source, function, and target that it is not particularly useful for the clinician to classify cytokines on the basis of the families discussed above. For example, activated macrophages in an inflammatory site synthesize growth factors, interleukins, interferons, and chemokines.

Both innate and adaptive responses result in the production of cytokines. T lymphocytes are the classic cytokine-producing cell of adaptive immunity, but macrophages, mast cells, and neutrophils also synthesize a wide range of cytokines upon stimulation. Cytokine interactions can be additive, combinatorial, synergistic, or antagonistic. Elimination of the action of a single molecule may have an unpredictable outcome; for example, monoclonal antibodies directed against TNF-α result in substantial suppression of immune responses but also increase susceptibility to multiple sclerosis. Finally, not only do innate and adaptive immune responses use cytokines as mediators and amplifiers of inflammation, but cytokines also modulate the initiation of immune responses; the function of most leukocytes is altered by preexposure to various cytokines. Thus, for many cytokines, their regulatory role may be as important as their actions as mediators of inflammation.

Table 1-3 Cytokines of Relevance to Ocular Immunology

Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.