Provided is a method for prevention and treatment of age-related macular degeneration through the suppression of cathepsin S expression, and retinal pigment epithelial cells in which cathepsin S is knocked down can be usefully used as a target for preventing or treating dry and wet age-related macular degenerations, in that the expression level of inflammatory cytokines and complement activity are reduced even under oxidative stress conditions, and the retinal pigment epithelial cells are not only more effective in suppressing complement activity than NF-B inhibitors, but also suppress intracellular neovascularization and tube formation.

As described herein, the activity of Treg cells can be mediated by cathepsin inhibition, e.g., in tumor environments, cathepsin inhibition results in increased Treg anti-tumor activity, while in non-tumor environments, cathepsin inhibition results in increased immunosuppressive activity. Accordingly, provided herein are methods of modulating Treg activity and methods of treating diseases (e.g., cancer or autoimmune diseases) by inhibiting cathepsins in Treg cells.

The present invention relates to a cathepsin inhibitor comprising or consisting of Formula (I) (X.sub.1)(X.sub.2)(X.sub.3)(X.sub.4)(Y)(X.sub.5)(X.sub.6)(X.sub.7) Formula (I) wherein (X.sub.1) is an amino acid selected from K, L, F and M; (X.sub.2) is an amino acid selected from L, A, D, E, F, G, H, I, K, M, N, S, W, Y, Q and R; (X.sub.3) is an amino acid selected from R, A and L; (X.sub.4) is an amino acid selected from M, I, K, V and Y; (X.sub.5) is an amino acid selected from P, A, I, L, V, W and Y; (X.sub.6) is an amino acid selected from K, A, E, F, G, I, L, M, N, Q, S, T, V, W, Y and Q; (X.sub.7) is an amino acid selected from D, A, E, F, G, I, L, M, N, P, T, V, W, Y, Q and R; and (Y) is a Michael acceptor.