Tertiary lymphoid tissues: a regional hub for kidney inflammation.

Tertiary lymphoid tissues (TLTs) are inducible ectopic lymphoid tissues that develop at sites of chronic inflammation in non-lymphoid organs. Same as lymph nodes, TLTs initiate adaptive immune responses and coordinate local tissue immunity. Although virtually ignored for decades, TLTs have recently received a great deal of attention for their ability to influence disease severity, prognosis and response to therapy in various diseases including cancer, autoimmune disorders and infections. TLTs are also induced in kidneys of patients with several CKDs such as IgA nephropathy and lupus nephritis. Nevertheless, TLTs in the kidney have not been extensively investigated, and their mechanism of development, functions and clinical relevance remain unknown, mainly because of the absence of adequate murine kidney TLT models and limited availability of human kidney samples containing TLTs. We recently found that aged kidneys, but not young kidneys, exhibit multiple TLTs after injury. Interestingly, although they are a minor component of TLTs, resident fibroblasts in the kidneys diversify into several distinct phenotypes that play crucial roles in TLT formation. Furthermore, the potential of TLTs as a novel kidney injury/inflammation marker as well as a novel therapeutic target for kidney diseases are also suggested. In this review article, we describe the current understanding of TLTs with a focus on age-dependent TLTs in the kidney and discuss their potential as a novel therapeutic target and kidney inflammation marker.