Cytokines​ are a broad and loose category of small ​proteins​ ​(5-20 kDa molecular weight) ​important in ​cell signaling​, particularly in the regulation of immune system responses to infection and inflammation. Cytokines​ are ​peptides​ and cannot cross the ​lipid bilayer​ of cells to enter the ​cytoplasm​. They communicate by signaling through cell surface receptors. Cytokines have been shown to be involved in autocrine, paracrine, and endocrine signaling pathways. ​Although secreted, cytokines are not considered to be hormones. ​There are several families of cytokines expressed in humans and other vertebrates and can act powerfully as individual molecular reagents although most often exert their influence combinatorially in multiple interactions c depending on the context in which they are expressed. ​The different types of cytokines, e.g., interferons, interleukins, lymphokines, chemokines, and tumor necrosis factors can act alone, work together, or work antagonistically in modulating a combinatorial response. Cytokines exert their influence on cells via cognate cell-surface receptors in either a paracrine fashion, i.e., by binding to receptors on other nearby cells, or in an autocrine mode, i.e., binding to receptors on the cell that secreted them.

Multiplex Measurement of Cytokine Expression

While familiar and reliable ​ELISA​ assays provide an excellent means of measuring the concentrations of individual cytokines, establishing expression profiles of multiple cytokines at once by multiplexing can be even more useful and informative during the development of a ​cell-based assay​. Multiplex measurements are technically challenging but can be accomplished by use of flow cytometers.

Cytokine families and their characteristics

Interferons (IFN)​ ​act by inhibiting viral replication. Infected cells produce and release interferons to alert uninfected cells to the presence of the virus.

Interleukins (IL)​ were initially thought to be expressed only by leukocytes (white blood cells) but were later seen to be expressed by other cell types. Different members of the family have pro-inflammatory or anti-inflammatory properties with essential roles in the activation and differentiation of immune cells. Lymphokines​ are produced by lymphocytes, the subset of leukocytes that includes T cells, B cells, and natural killer (NK) cells. Typically produced by T cells to coordinate immune system responses, lymphokines have multiple functions, including the attraction of macrophages to an infected site. Lymphokines also participate in the production of antibodies by B cells.

Chemokines ​are chemotactic cytokines. Their function is to act as a chemoattractant in guiding the migration of cells in following a signal of increasing chemokine concentration toward its source.

Tumor necrosis factors (TNF)​ ​are transmembrane proteins expressed predominantly by immune cells that are released by extracellular enzymatic cleavage to function as cytokines in immune response and inflammation.

Cytokine storms and Covid-19

As part of the current COVID-19 pandemic, there has been much mention of the lethality of so-called cytokine storms. Cytokine storms are particularly interesting because they can occur in young “healthier” patients that otherwise should recover better from SARS-Cov-2 infection. Cytokine storms represent a severe immune reaction in which too many cytokines are released into the blood too quickly. However, these are not necessarily just inflammatory cytokines, anti-inflammatory cytokines are also involved. Medical science continues to work to understand how to best treat these dangerous overproductions of cytokines that are often part of the body’s response to sepsis. Cytokine storms can occur not only as the result of an infection, but also because of autoimmune conditions or after some types of immunotherapy treatment. Some cytokines trigger apoptosis, i.e., programmed cell death, that can be a protective mechanism to prevent the spread of a disease. But if apoptosis begins to affect too many cells, significant tissue death can result – lung tissue in the case of COVID-19. Alveoli leak and fill with fluid, leading to pneumonia and dangerously low blood oxygen levels that can then result in organ failure elsewhere in the body.

Cytokines as biomarkers

Although cytokines are known to possess many diverse and important functions, they often serve as valuable biomarkers in research that may not be focused directly on their roles in either inflammation or immune responses. A biomarker is a molecule whose measurable levels can be informative as to the success of an experimental approach or therapeutic treatment because of previous observations that particular levels of that molecule could be associated with certain outcomes. In such a situation, it may not be important to know why the biomarker went up or down, only that quantitating its presence is helpful in interpreting the overall result of the experiment or treatment.

MarinBio scientists are experts in molecular biomarker analysis​​.

In the case of cytokines, researchers may attempt to associate particular expression profiles of multiple biomarkers with various outcomes, especially when the measurement of the levels of individual cytokines, e.g., by ELISA, is not sufficiently instructive. For this reason, multiplex measurements of a large number of cytokines using more advanced technology is often desirable.