Question

A fungus produces a chemical signal that decreases expression of CD28 and increases expression of CTLA4?

Answer 1

*Cryptococcus neoformans produces Black melanin-like pigments which are virulent in nature , it act as a  signal that decreases expression of CD28 and increases expression of CTLA4.

*These are known as cryptococcal antigens.

*T-cell activation is mediated by antigen-specific signals from the TCRζ/CD3 and CD4–CD8–p56lck complexes in combination with additional co-signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death (PD-1), and others. CD28 and ICOS provide positive signals that promote and sustain T-cell responses, while CTLA-4 and PD-1 limit responses. The balance between stimulatory and inhibitory co-signals determines the ultimate nature of T-cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity.

*Both CD28 and CTLA-4 engage the same ligands, B7-1 (CD80) and B7-2 (CD86), on antigen-presenting cells; however, unlike that of CD28, CTLA-4 ligation to B7 results in down-regulation of the adaptive immune response, i.e., inhibition of IL-2 production, IL-2R expression, and T-cell proliferation thus lowering the protective response against this fungi.