Lymphoid neogenesis: de novo formation of lymphoid tissue in chronic inflammation through expression of homing chemokines.

Chronic inflammation is a complex pathophysiological process with accumulation of mononuclear cells seen in response to invading pathogens, neoplastic transformation, or autoimmune recognition of self-antigens. The inflammatory process has evolved to facilitate effective elimination of pathogens and tumors and it is normally transient and turned off when the causative stimulus has been eliminated. Occasionally, however, the process is sustained for a long time and can lead to severe tissue damage. This is seen in organ-specific autoimmune diseases such as rheumatoid arthritis, Sjögren's syndrome, and Hashimoto's thyroiditis, but also in infectious diseases such as Helicobacter pylori-induced gastritis. Disturbingly, many of these chronic inflammatory diseases are associated with an increased risk for neoplastic transformation and development of lymphomas. This review summarizes experimental evidence suggesting that chronic inflammation involves ectopic de novo formation of organized lymphoid tissue and that this lymphoid neogenesis is regulated by expression of homing chemokines.