Kinetic-segregation model of T cell activation

Kinetic-segregation model of T-cell activation is one of the TCR signaling models. It offers an explanation for how TCR binding to its ligand triggers T-cell activation, based on size-sensitivity for the molecules involved. Simon J. Davis and Anton van der Merwe, University of Oxford, proposed this model in 1996. Kinetic-segregation model was originally used to elucidate how binding of TCR to peptide-MHC-complex on APC is communicated across the T-cell membrane resulting in T-cell activation. However, its authors suggest it might be also applied to CD28 triggering by superagonistic antibodies (mitogenic antibodies).

Applications of the proposed mechanism

TCR triggered by peptide-MHC binding

TCR signals by stimulating tyrosine phosphorylation. In the resting T-cell, the molecules involved in this process are repeatedly colliding by means of diffusion. The TCR/CD3 complex is constantly being phosphorylated by Lck (membrane associated tyrosine kinase) and in turn continuously dephosphorylated by CD45 (tyrosine phosphatase). It all happens in a random manner and as a result the overall phosphorylation of TCR is low. T-cell activation does not proceed. [1]

The TCR/peptide-MHC complex, formed when a T-cell recognizes its ligand and the T-cell-APC contact occurs, spans a short length. As the authors propose, this forms small zones of close contact, from which the inhibitory CD45 phosphatase molecules with ectodomains too large to fit in are excluded. CD148 is another large-ectodomain phosphatase excluded from the close-contact zone. [1]

CD45 steric exclusion extends the phosphorylation half-lives of TCR/peptide-MHC complexes, which are trapped within the close-contact zone. Such prolonged phosphorylation of ITAMs by Lck kinase allows time for ZAP-70 recruitment, its activation by phosphorylation and subsequent phosphorylation of adaptor proteins LAT and SLP-76, all known as downstream signaling steps. Full T-cell activation is initiated by multiple triggering events described above. When T-cell and APC membranes separate, the close-contact zone vanishes and large-ectodomain tyrosine phosphatases are allowed to restore the ground state.

Phosphorylation of TCR/CD3 complexes that entered the close-contact zone but did not bind their peptide-MHC ligand is too short-lived to induce signaling because they rapidly diffuse out of the segregation zone back to the vicinity of inhibitory tyrosine phosphatases. Unbound complexes are not held in the zone of contact interface and do not trigger T-cell activation.

Antibody-induced signaling by CD28

In the resting T-cell there is no net phosphorylation of CD28 (one of the molecules providing co-stimulatory signals required for T-cell activation). Kinetic-segregation model uses here the same explanation as it provides for low net phosphorylation of TCR in the resting T-cell described previously.

Binding of both conventional and superagonistic (mitogenic) antibodies in suspension does not constrict the dephosphorylation effect of phosphatases acting on CD28. However, when these antibodies are immobilized (either by secondary antibody bound to plastic or by Fc receptors on other cells) considerable steric constraints emerge. It is of note, that the immobilized conventional antibody poses less prominent spatial constraints than the immobilized superagonistic antibody. CD45 phosphatase is not completely excluded from the close-contact zone and thus the signal generated in the case of a conventional antibody is weaker. Immobilized superagonistic antibodies bound to CD28 exclude CD45 phosphatases completely and the signal leading to T-cell activation is stronger.

Further applications

The tyrosine kinase Lck functions either in conjunction with a co-receptor molecule (CD4 or CD8) or as a free Lck kinase. The kinetic-segregation model might be applied to both co-receptor dependent and co-receptor independent signaling through TCR.

Other triggering models

According to the primary mechanism they propose for signal transduction and the fate of TCR/CD3 complex in T-cell triggering, the TCR signaling models might be divided into three basic categories. They are classified as aggregation models, conformational change models and segregation models. These models are not strictly mutually exclusive and T-cell triggering may actually combine various mechanisms.[1]

Aggregation models

Aggregation models propose that the aggregation of TCR/CD3 upon binding a ligand results in the proximity of tyrosine kinases, leading to ITAM phosphorylation necessary for further signal transduction.[1]

Conformational change models

In general, conformational change models postulate that binding of peptide-MHC alters TCR/CD3 complex by exerting a mechanical pulling force. This is supposed to result in conformational changes in the cytoplasmic tails of CD3 molecules, thus resulting in signal transduction.[1]

Segregation models

Kinetic segregation model proposing size-based exclusion of inhibitory molecules from the contact site is not the only segregation model suggested for T-cell activation. Lipid raft association model suggests that segregation of TCR/CD3 complexes in lipid rafts enriched in Lck tyrosine kinases and other signaling molecules and depleted of CD45 tyrosine phosphatases might provide an environment favoring TCR triggering.[2]

References
  1. Choudhuri, Kaushik; Kearney, Alice; Bakker, Talitha R.; van der Merwe, P.Anton (24 May 2005). "Immunology: How Do T Cells Recognize Antigen?". Current Biology. 15 (10): R382–R385. doi:10.1016/j.cub.2005.05.001. ISSN 0960-9822. PMID 15916940.
  2. Lanzavecchia, Antonio; Iezzi, Giandomenica; Viola, Antonella (8 January 1999). "From TCR Engagement to T Cell Activation: A Kinetic View of T Cell Behavior". Cell. 96 (1): 1–4. doi:10.1016/S0092-8674(00)80952-6. ISSN 0092-8674. PMID 9989490.
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