Infectious agents as causes of non-Hodgkin lymphoma.

Among exposures presently viewed as possible etiologic factors in non-Hodgkin lymphoma (NHL), infections are close to being regarded as established causes. Infectious agents causing NHL can be classified, according to mechanism, into three broad groups. First, some viruses can directly transform lymphocytes. Lymphocyte-transforming viruses include Epstein Barr virus (linked to Burkitt's lymphoma, NHLs in immunosuppressed individuals, and extranodal natural killer/T-cell NHL), human herpesvirus 8 (primary effusion lymphoma), and human T lymphotropic virus type I (adult T-cell leukemia/lymphoma). Second, human immunodeficiency virus is unique in causing profound depletion of CD4+ T lymphocytes, leading to acquired immunodeficiency syndrome and an associated high risk for some NHL subtypes. Third, recent evidence suggests that some infections increase NHL risk through chronic immune stimulation. These infections include hepatitis C virus as well as certain bacteria that cause chronic site-specific inflammation and seem to increase risk for localized mucosa-associated lymphoid tissue NHLs. Establishing that an infectious agent causes NHL depends on showing that the agent is present in persons with NHL as well as laboratory experiments elucidating the mechanisms involved. Only epidemiologic studies can provide evidence that infection is actually a risk factor by showing that infection is more frequent in NHL cases than in controls. Given the range of mechanisms by which infections could plausibly cause NHL and our growing molecular understanding of this malignancy, this field of research deserves continued attention.