Costimulation through CD28 enhances T cell-dependent B cell activation via CD40-CD40L interaction

Area of Science:

• Immunology
• Cellular immunology
• T cell and B cell interactions

Background:

• T cell activation is critical for adaptive immunity.
• Costimulatory signals modulate T cell responses.
• T cell help is essential for B cell activation, proliferation, and antibody production.

Purpose of the Study:

• To investigate the role of CD28 costimulation in T cell helper function.
• To determine the mechanism by which anti-CD28 enhances T cell-dependent B cell responses.
• To elucidate the dependence of T cell help on CD40 ligand (CD40L) expression.

Main Methods:

• Activation of T cells with anti-CD3 and anti-CD28 monoclonal antibodies (mAbs).
• Analysis of T cell helper function and cytokine production (IL-2, IL-4).
• Assessment of CD40 ligand (CD40L) expression on T cells.
• Evaluation of B cell proliferation and immunoglobulin secretion.
• Inhibition studies using soluble CD40 fusion proteins and cyclosporin A.
• Studies with hyper IgM syndrome patients lacking functional CD40L.

Main Results:

• Anti-CD28 costimulation augmented T cell-dependent B cell growth and differentiation.
• Anti-CD28 enhanced IL-2 and IL-4 production but did not solely rely on soluble lymphokines.
• CD28 costimulation increased CD40L expression on T cells.
• CD40L expression was strictly required for initiating T cell-dependent B cell responses.
• Both CD40L expression and T cell help were blocked by cyclosporin A and remained sensitive during CD28 costimulation.
• Anti-CD28 could not rescue the T cell helper deficiency in hyper IgM syndrome patients.

Conclusions:

• Anti-CD28-induced T cell help for B cells is primarily mediated through a CD40L-dependent mechanism.
• CD40L expression on T cells is essential for initiating T cell-dependent B cell responses.
• Reciprocal signaling between T and B cells, involving CD40 and CD28 ligands, may amplify immune responses.