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A point mutation in CD28 distinguishes proliferative signals from survival signals
1Department of Immunology, University of Toronto, Ontario M5S 1A2, Canada.
Abstract:
Upon interaction with its ligand, B7, CD28 becomes phosphorylated on tyrosines. One tyrosine in particular (Y170 in mouse CD28, Y173 in human CD28) has received much attention. This is because it permits CD28 to recruit SH2-containing signaling molecules, including phosphoinositide 3 kinase, Grb2 and Gads. Using mice we employed a transgenic approach to express a tyrosine-->phenylalanine mutant form of CD28 that uncouples these SH2-mediated interactions from CD28. The CD28 mutant is unable to up-regulate expression of the prosurvival protein Bcl-xL, rendering the T cells more susceptible to radiation-induced death. Nonetheless, this mutated form of CD28 still prevents the induction of anergy and promotes T cell proliferation, interleukin 2 secretion and B cell help. Thus, we describe a single point mutation within the CD28 cytoplasmic domain that uncouples signals required for proliferation and survival.
Insights
A single mutation in CD28 (an immune cell receptor) separates its roles in T cell proliferation from its role in cell survival. This finding helps understand T cell signaling pathways.
Area of Science:
- Immunology
- Molecular Biology
- Cell Signaling
Background:
- CD28 is a crucial T cell co-stimulatory receptor that interacts with B7 ligands.
- Phosphorylation of CD28 on specific tyrosine residues, particularly Y170/Y173, enables recruitment of SH2-domain containing signaling molecules.
- These recruited molecules, such as PI3K and Grb2, are critical for downstream signaling events.
Purpose of the Study:
- To investigate the distinct roles of CD28 signaling in T cell function.
- To determine if specific tyrosine phosphorylation sites on CD28 mediate differential signaling outcomes.
- To dissect the signaling pathways governing T cell proliferation, survival, and anergy.
Main Methods:
- Utilized a transgenic mouse model expressing a CD28 mutant (tyrosine to phenylalanine) to disrupt SH2-mediated interactions.
- Analyzed T cell responses, including proliferation, cytokine secretion (IL-2), B cell help, and susceptibility to radiation-induced death.
- Assessed the induction of T cell anergy in the presence of the CD28 mutation.
Main Results:
- The CD28 mutant failed to up-regulate the prosurvival protein Bcl-xL, increasing T cell susceptibility to radiation-induced death.
- Despite impaired survival signaling, the CD28 mutant effectively prevented T cell anergy induction.
- The mutated CD28 still supported T cell proliferation, interleukin-2 secretion, and B cell help.
Conclusions:
- A single point mutation in the CD28 cytoplasmic domain can uncouple signals required for T cell proliferation and survival.
- Specific tyrosine phosphorylation sites on CD28 differentially regulate distinct T cell functions.
- This highlights the complexity of CD28 signaling and its role in balancing T cell activation and homeostasis.