Interleukin 15-like (IL-15L) is an
interleukin
Interleukins (ILs) are a group of cytokines (secreted proteins and signal molecules) that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. The human genome encodes more than 50 interleukins and related ...
, a type of
cytokine
Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling.
Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
signaling in the
immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
. It is a secreted protein of approximately 120 amino acids
and related to
IL-2
The Ilyushin Il-2 (Russian language, Russian: Илью́шин Ил-2) is a Ground attack aircraft, ground-attack plane that was produced by the Soviet Union in large numbers during the World War II, Second World War. The word ''shturmovík'' (C ...
and
IL-15.
Intact ''IL-15L'' genes are found in bony fish,
cartilaginous fish,
reptiles, and many mammals.
However, in humans and mice only an ''IL-15L''
pseudogene
Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Pseudogenes can be formed from both protein-coding genes and non-coding genes. In the case of protein-coding genes, most pseudogenes arise as superfluous copies of fun ...
is found.
The location of the ''IL-15L'' gene in the genome is conserved from sharks to mammals and on a different chromosome than ''IL-2'' and ''IL-15''.
Bony fish (trout) IL-15L specifically stimulates the expression of type 2 immunity cytokines IL-4/13A and IL-4/13B (homologs of
IL-4 and
IL-13.), probably by stimulating
ILC2
ILC2 cells, or type 2 innate lymphoid cells are a type of innate lymphoid cell. Not to be confused with the ILC. They are derived from common lymphoid progenitor and belong to the lymphoid lineage. These cells lack antigen specific B or T cell r ...
cells;
this is quite different from the functions found for
IL-2
The Ilyushin Il-2 (Russian language, Russian: Илью́шин Ил-2) is a Ground attack aircraft, ground-attack plane that was produced by the Soviet Union in large numbers during the World War II, Second World War. The word ''shturmovík'' (C ...
and
IL-15,
including in fish.
For mammalian IL-15L, a function has not been found yet. In both fish and mammals, IL-15L seems to be quite dependent for its stability on “heterodimer” formation with
IL-15Rα,
and functions for fish IL-15L could only be found if partnered with (presented “in trans” by)
IL-15Rα
It is unclear why many mammals, and also birds and amphibians,
lost IL-15L. It has been postulated that in fish the IL-15L cytokine resides at the beginning of a type 2 immune cascade,
a function that in mammals is performed by cytokines such as
TSLP
Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-2-like cytokine, alarmin, and growth factor involved in numerous physiological and pathological processes, primarily those of the immune system. It shares a common ancestor with IL-7. ...
,
IL-25, and
IL-33
Interleukin 33 (IL-33) is a protein that in humans is encoded by the ''IL33'' gene.
Interleukin 33 is a member of the IL-1 family that potently drives production of T helper-2 (Th2)-associated cytokines (e.g., IL-4). IL33 is a ligand for ST2 ...
; none of these three cytokines have been found in fish,
and their evolution in tetrapod species may have made IL-15L redundant.
References
{{reflist
Interleukins