Pharmaceutical compositions comprising a compound selected from the group consisting of Compound Nos. 1, 2, 3, 4, 5, 6, 7, 13, 22, 23, 24 and 25, which are described in Table 1, and a pharmaceutically acceptable excipient. The pharmaceutical composition of the invention may further comprise an antigen, and/or an adjuvant. Also provided are methods of inhibiting a regulatory T (Treg) cell-mediated immune suppression, or more generally a method for enhancing immune response using a pharmaceutical composition comprising a ligand for human Toll-like receptor (TLR) 8 which activates the MyD88-IRAK4 signalling pathway. The present invention further provides a method of screening for an inhibitor of Treg cells suppressive activity of host immune response using CD4+ Treg cells which express CD25, GITR and FoxP3 secrete IL-10, and are able to suppress the activation of CD4+ T cells.
