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 rela ...
, a type of
cytokine
Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in a ...
signaling in the
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
. It is a secreted protein of approximately 120 amino acids
and related to
IL-2
The Ilyushin Il-2 (Russian: Илью́шин Ил-2) is a ground-attack plane that was produced by the Soviet Union in large numbers during the Second World War. The word ''shturmovík'' (Cyrillic: штурмовик), the generic Russian term ...
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. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by reverse transcription of an mRNA transcript. Pseudogenes are ...
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: Илью́шин Ил-2) is a ground-attack plane that was produced by the Soviet Union in large numbers during the Second World War. The word ''shturmovík'' (Cyrillic: штурмовик), the generic Russian term ...
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 a protein belonging to the cytokine family. It is known to play an important role in the maturation of T cell populations through activation of antigen-presenting cells.
TSLP is produced mainly by non-hem ...
,
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