Viral Infections in Hematopoietic Stem Cell Transplant Recipients

Viral infections are important as causes of morbidity and mortality after allogeneic stem cell transplantation (SCT). Severe viral infections are more common after unrelated and mismatched donor SCT and in particular after haploidentical SCT. B-cell function and specific antibodies are the main defense mechanisms against infection with exogenous viruses, thus reducing the risk for reinfection in already seropositive individuals. On the other hand, T-cell function in particular cytotoxic T-cell function is the main mechanism for preventing severe viral disease and also for the control of viruses such as herpesviruses that can cause latency and thus reactivate in an immunocompromised individual. The immune defects in SCT-patients are frequently complex with defects in cytotoxic T-lymphocyte, helper T-lymphocyte, NK-cell, and B-lymphocyte functions. T-cell dysfunction is usually most important early after SCT while deficient B-cell reconstitution can remain for many years after SCT. Furthermore, since loss of specific antibodies occurs frequently over time after allogeneic SCT, this will also increase the risk for reinfections with previously encountered viruses such as measles or varicella-zoster virus (VZV) and allow reactivation of viruses controlled by antibodies such as hepatitis B virus (HBV) [1, 2].