Is the exquisite specificity of lymphocytes generated by thymic selection or due to evolution?

Area of Science:

• Immunology
• Evolutionary Biology
• Computational Biology

Background:

• Lymphocyte antigen receptors (T and B cells) require specific binding properties to balance immune recognition and self-tolerance.
• Previous work suggested optimal receptor specificity is inversely related to the number of self-antigens.
• T cells also increase specificity during thymic negative selection due to deletion of cross-reactive clones.

Purpose of the Study:

• To mathematically model and analyze the combined impact of evolutionary and thymic selection on lymphocyte specificity.
• To investigate how varying binding probabilities in the pre-selection repertoire influence post-selection specificity.
• To understand the role of self-antigen load in shaping both pre- and post-selection lymphocyte repertoires.

Main Methods:

• Extension of a pre-existing mathematical model for lymphocyte receptor specificity.
• Inclusion of T cells with diverse binding probabilities in the initial repertoire.
• Analysis of how negative selection impacts receptor cross-reactivity and repertoire composition.

Main Results:

• Negative selection effectively removes highly cross-reactive T cell clonotypes.
• Post-selection T cell specificity is influenced by the number of self-antigens, independent of evolutionary optimization.
• The optimal pre-selection repertoire specificity is also dependent on the self-antigen load.

Conclusions:

• Both evolutionary pressures and thymic negative selection contribute significantly to the high specificity of T lymphocytes.
• Increased self-antigen load necessitates more specific pre-selection repertoires to prevent excessive T cell loss.
• This dual mechanism ensures robust immune responses essential for species survival.