The immunomodulatory properties of low-molecular tri- and tetrapeptides.

Immunomodulatory effects of six linear tri- or tetrapeptides were studied. These peptides are physiologically inert precursors, which under the action of native proteases split into a C-terminal dipeptide ester, which is subsequently converted by spontaneous intramolecular cyclization to a biologically active compound, i.e., a spirocyclic dipeptide. The following biological activities were evaluated using human lymphocytes: recovery of receptors for sheep red blood cells, test of active E-rosettes, and modulation of T-cell mitogen responses. All tested peptides revealed significant inhibition in some assay, although none of them induced significant inhibition in all assays. The compounds, I, IV, and VI proved to be the best inhibitors in comparison with Alaptide, i.e., cyclo(Ala-Acp).