A 51-year-old man is seen in clinic because he feels chronically ill. Physical examination is notable for a palpable, nodular liver. Serum studies for hepatitis virus markers are positive for anti-HCV and negative for HbsAg and HBcAg. Core biopsy of the liver is performed to evaluate the extent of his disease, and demonstrates chronic hepatitis with severe activity and severe fibrosis. The patient is treated with interferon for 6 months, but fails to respond. His condition continues to deteriorate, and he is treated with ohotopic liver transplantation. One year after ohotopic liver transplantation, the man develops rising transaminase and bilirubin levels. In order to minimize chronic rejection injury to hepatic endothelial cells, immunosuppressive therapy should be aimed at downregulating which of the following components of the immune response?