Parasitic nematodes can downregulate the immune response of their hosts through the induction of immunoregulatory cytokines such as interleukin-10 (IL-10). To define the underlying mechanisms, we measuredin vitro the production of IL-10 in macrophages in response to cystatin fromAcantho-cheilonema viteae, an immunomodulatory protein of filarial nematodes, and developed mathematical models of IL-10 regulation.