Cytokine receptors

Receptor classification

Based on the structure of their intra- and extracellular domains, seven basic classes are distinguished:

1. Hematopoietin (Class I): Characterized by the presence of 2–7 extracellular domains, 4 cysteine residues, and a unique WSXWS motif. They recognize most interleukins (IL-2, 3, 4, 6, etc.), as well as erythropoietin and colony-stimulating factors.
2. Interferon receptors (Class II): Have 1–2 domains with cysteine residues, but lack the WSXWS sequence. They bind interferons and IL-10.
3. TNF family (Class III): Form trimers upon binding to ligands (e.g., TNFα).
4. IL-1 receptors: The extracellular portion belongs to the immunoglobulin superfamily. The intracellular TIR domain is analogous to that of Toll-like receptors (TLRs), resulting in similar signal transduction.
5. Immunoglobulin-like: Have 5 Ig-like extracellular domains. An important feature is intrinsic tyrosine kinase activity (c-Fms, c-Kit, Flt-3 kinases). These kinases are proto-oncogenes.
6. Chemokine receptors (serpentines): The polypeptide chain spans the membrane 7 times. They transmit signals via G proteins.
7. Other receptors: Atypical structures (e.g., for TGFβ).

Subunit composition and affinity changes

A crucial principle of the cytokine network is the presence of shared polypeptide chains that are part of receptors for different cytokines. For example, the shared γ-chain participates in the recognition of IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. A defect in its gene leads to severe immunodeficiencies.

Receptor affinity depends on how fully "equipped" it is with chains. This is clearly seen in the example of the IL-2 receptor:

• Low-affinity (α-chain only): binds the cytokine but does not transmit a signal. Kd = 10⁻⁸ M.
• Intermediate-affinity (βγ dimer): present on resting and NK cells. Capable of signal transduction in NK cells. Kd = 10⁻⁹ M.
• High-affinity (αβγ trimer): expressed on activated T cells. Includes Jak1 and Jak3 kinases. Kd = 10⁻¹⁰ M.

Upon activation, cells sharply increase the number of receptors and "complete" their assembly. Thus, the synthesis of the α-chain allows the formation of the high-affinity IL-2 trimer, ready for efficient function.

Jak-STAT signaling pathway

The main intracellular pathway for cytokine (hematopoietin) receptors is mediated through Jak enzymes (Janus kinases) and STAT transcription factors.

The activation mechanism includes the following stages:

1. Cytokine binding causes receptor dimerization (approximation of subunits).
2. Receptor-associated Jak tyrosine kinases are activated via autocrine phosphorylation.
3. Jak kinases phosphorylate the intracellular regions of the receptor itself.
4. Inactive STAT monomers from the cytoplasm attach to these regions via their SH2 domains.
5. Jak kinases phosphorylate STAT factors, after which the latter form dimers.
6. STAT dimers translocate to the nucleus, where they initiate the expression of target genes (adhesion molecules, enzymes, or cell cycle regulators).

There is specificity: for example, STAT1 is responsible for the signal from IFNγ, while STAT4 is responsible for IL-12.

Internalization and negative regulation

Following interaction with the receptor, the complex is taken up by the cell (internalized) and degraded in endosomes. This is necessary not for signal transduction, but for cytokine utilization to prevent its excessive accumulation.

Proteins of the SOCS family (Suppressors of cytokine signaling) are responsible for halting the cascades. They are activated by the cytokine signals themselves, forming a negative feedback loop.

Using their SH2 domain, SOCS factors operate in three ways: they directly inhibit and dephosphorylate Jak kinases, compete with STAT proteins for binding sites on the receptor, and direct signaling proteins for degradation. Disruption of SOCS genes leads to an excess of IFNγ, lymphopenia, and massive apoptosis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Alternative signaling pathways (Src kinases, PI3K, and AP-1 factor)
• Detailed analysis of shared subunits (γ, β, α) and their role in the cytokine network
• Specificity of Jak kinases and STAT factors for various groups of interleukins
• Clinical consequences of defects in shared chain genes and Jak3 kinase
• The role of c-Fms, c-Kit, and Flt-3 kinases as proto-oncogenes

Other topics in the "Innate Immunity" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.