The tumor-draining lymph node as an immune-privileged site.

Lymph nodes that lie immediately downstream of tumors [tumor-draining lymph nodes (TDLNs)] undergo profound alterations due to the presence of the upstream tumor. The antigen-presenting cell population in TDLNs becomes modified such that tumor-derived antigens are cross-presented by host cells in a tolerizing fashion. In addition, the number and suppressor activity of regulatory T cells (Tregs) are increased in the TDLN. Emerging evidence suggests that some of these Tregs may be generated de novo against specific tumor-derived antigens, and thus they arise as a direct consequence of antigen presentation in the TDLN. Others may represent Tregs against self-antigens, which undergo preferential activation in the tolerogenic milieu of the TDLN. The TDLN thus becomes an anatomic context in which presentation of new antigens not only fails to elicit a protective immune response but also actively creates systemic tolerance. In this regard, the TDLN displays features analogous to classical immune privilege. Accumulating evidence thus suggests that the TDLNs, although small in size, may exert a profound tolerizing influence on the rest of the immune system. These mechanisms will need to be interrupted in order for clinical anti-tumor immunotherapy to be successful.