Role of secondary lymphoid tissues in primary and memory T-cell responses to a transplanted organ.

Secondary lymphoid tissues are the hub of adaptive immune responses wherein rare cognate lymphocytes encounter dendritic cells bearing antigen from peripheral tissues and differentiate into effector and memory cells that eliminate antigen. It is accepted that immune responses against microbial and tumor antigens are initiated within secondary lymphoid tissues. There is less agreement on whether the same principle applies to immune responses to a transplanted organ because an allograft expresses foreign major histocompatibility complex and contains donor antigen presenting cells that could activate T cells directly in situ leading to rejection. Recent studies confirm that although naïve T cells can be primed within the allograft, their differentiation to effect rejection is dependent on secondary lymphoid tissues. Antigen-experienced memory T cells, unlike Naïve T cells, function largely independent of secondary lymphoid tissues to cause allograft rejection. In an alloimmune response, secondary lymphoid tissues support not only immune activation but also immune regulation essential for allograft survival. Here, we will review recent findings and discuss the role of secondary lymphoid tissues in primary and memory alloimmune responses.