IntroductionThe geographic heterogeneity in the incidence of B-cell nonHodgkin lymphomas (NHLs) suggests that environmental factors such as infections might have a role in lymphomagenesis. 1,2 Because of the inherent genetic instability of lymphocytes, lymphoid proliferation increases the risk of transformation, and sustained activation of the lymphoid system, which can be observed during chronic infection, immunodeficiency, and autoimmunity, constitutes a risk factor for lymphomas. [3][4][5] Congenital and acquired immunodeficiencies associated with HIV infection and solid organ or hematopoietic transplantation increase the risk of developing B-cell NHLs. 6,7 Similarly, Sjögren syndrome and other autoimmune conditions are also associated with an increased risk of lymphomas. 8,9 Certain types of lymphomas are associated with specific microbial infections, and infection-associated lymphomas currently fall in diverse histopathologic categories (Table 1). 10 Infections may contribute to lymphomagenesis by promoting favorable conditions for lymphocyte transformation, such a increased proliferation or decreased apoptosis of lymphoid cells. 11 Direct lymphocyte transformation by a given microbial agent is the simplest scenario accounting for infection-associated lymphomas. Lymphotropic transforming viruses such as Epstein-Barr virus (EBV), human herpesvirus 8 (HHV8), and human Tlymphotropic virus 1 (HTLV-1) directly infect a subset of lymphoid cells in which they express viral oncogenes. [12][13][14] An alternative scenario to direct transformation of lymphocytes has more recently emerged for microbial species associated with lymphomas but that do not directly infect or transform lymphoid cells. They have in common the ability to persist chronically in host tissues and trigger a sustained lymphoid proliferation, giving a selective advantage to lymphoid clones that still remain dependent upon antigen stimulation. 15,16 According to this model, the microbial pathogen is neither intrinsically transforming nor oncogenic, but can be viewed as a chronic source of antigens increasing the proliferative rate of lymphoid effectors, hence fueling the transformation process. This model has emerged with the description of several lymphomas developing in the context of chronic antigen-dependent immune stimulation, among which H pylori-associated gastric mucosa-associated lymphoid tissue (MALT) lymphoma is the best characterized 15,17,18 (Figure 1). Precise elucidation of the mechanisms underlying this "indirect" lymphomagenesis as well as completion of the inventory of the microbial species driving these antigen-dependent lymphoproliferations may provide important clues for their early diagnosis and the rationalization of the therapeutic interventions for this subtype of lymphomas.This group of lymphomas often involves extranodal sitesnormally devoid of organized lymphoid tissue-and manifests initially as indolent low-grade proliferations, reminiscent of the normal lymphoid hyperplasia driven by a physiologic antigenic stimulation....