TLR2 agonists enhance CD8⁺Foxp3⁺ regulatory T cells and suppress Th2 immune responses during allergen immunotherapy

Pam3CSK4, a synthetic TLR2 ligand, has been shown to expand CD4⁺ regulatory T cells (Treg cells). Less is known about the function of CD8 ⁺ Treg cells than about the function of CD4⁺ Treg cells generated during allergen-specific immunotherapy (IT). This study investigated whether Dermatophagoides pteronyssinus-specific IT could expand the CD8 ⁺CD25⁺Foxp3⁺ Treg population and whether Pam3CSK4 could enhance the Treg population. PBMCs were isolated from healthy control subjects and from mite-sensitive asthmatic patients during IT at three specific times: before IT and 6 mo and 1 y after the maximum-tolerated dose. This study was performed without a placebo-controlled group. D. pteronyssinus-specific IT induced a significant increase in CD8 ⁺Foxp3⁺ Treg cells expressing intracellular IL-10 and granzyme B. Costimulation of PBMCs with Pam3CSK4 and D. pteronyssinus 2 expanded the CD8⁺CD25⁺Foxp3⁺ Treg population and inhibited D. pteronyssinus 2-induced IL-4 production. Pam3CSK4-treated CD8 ⁺CD25⁺ Treg cells directly suppressed CD4⁺ T cell proliferation by cell-contact inhibition. TUNEL revealed that CD8 ⁺CD25⁺ Treg cells, but not CD4⁺CD25⁺ Treg cells, directly induced CD4⁺CD45RO^hi+ apoptosis. Our results provide direct evidence that Pam3CSK4 induces an immunomodulatory effect by inducing CD8⁺ Treg cells; therefore, it may be a good adjuvant for the treatment of mite allergies.