Although the remarkable efficacy of biological therapy has resulted in significant success in inflammatory bowel disease (IBD) management,susceptibility to infections remains a concern. The biological agents include the tumor necrosis factor-a (TNF-a) inhibitors, for instanceinfliximab,andotherimmunomodulatingagents,suchasnatalizumab.Progressivemultifocalleukoencephalopathy(PML),ararebutmostlyfatalopportunistic brain infection caused by reactivation of the human polyomavirus JC virus (JCV), has been found in two patients with multiplesclerosis and one patient with Crohn’s disease (CD), linked to treatment with natalizumab. After these cases of PML, the commercial andinvestigational use of natalizumab was suspended in February 2005 but was subsequently resumed for multiple sclerosis and for CD, onlythroughaspecialrestricteddistributionprogram.Thisreview,startingfromanextensiveliteraturesearchbythePubMeddatabase,resumestheclinicalaspectsandpathophysiologyofCDandfocusesonthebiologicsincurrentuseinCD(infliximab,adalimumab,andnatalizumab),inordertoprovideareferenceandgatewaytoprevention,recognition,andmanagementofJCV,intheearlyyearsofbiologicalagentstherapy.Italso proposed to provide an overview on the hypothetical mechanism of reactivation of JC virus related to the use of these drugs.J. Cell. Physiol. 224: 316–326, 2010. 2010 Wiley-Liss, Inc.Crohn’s disease (CD) is a chronic inflammatory bowel disease(IBD)ofunknownetiologythatresultsinsignificantmorbidityandhealth carecosts. Morethan400,000peoplein theUnitedStateshave CD. The prevalence continues to increase because of itsrising incidence and improved survival (Loftus and Sandborn,2002;ComerfordandBickston,2004).Thechronicnatureoftheillness causes frequent hospitalizations; most patients eventuallyrequire surgery secondary to complications, such as strictures,abscesses, fistula, or refractory disease. The disease seems tooccur when the intestinal immune cascade is triggered by anantigeningeneticallysusceptibleindividuals.Overactivationoftheenteric immune and inflammatory pathways causes mucosaldamage resulting in the clinical signs and symptoms. Variousmedications, including 5-aminosalicylates, antibiotics,corticosteroids, and immunomodulators, such as purineantimetabolites and methotrexate, have traditionally been usedtocontrolinflammation.Theiruseisintendedtopreventsurgeryandimprovethepatient’squalityoflife.Nonecurethediseaseandunfortunately, many patients require steroids to control theirsymptoms. A wide range of dose-related adverse effects makesthis an unappealing strategy. Immunomodulators are effectivemaintenance drugs, but have a slow onset of action with clinicalremission rates of approximately 40%. The limitations ofconventionalagentsleavetheclinicianinneedofamedicationthatissteroid-sparing,quickacting,andmaintainsremission(Hanaeuret al., 2002).Recently, biological therapy has brought a paradigm shift inthemanagementofIBD.Theintroductionofbiologicagentshasproduced an astounding transformation by halting or slowingthe progression of IBD, rheumatoid arthritis (RA), psoriaticarthritis and multiple sclerosis (MS) resulting in markeddecrease of disability and improvement in quality of life andhealth outcomes (Saketkoo and Espinoza, 2006). Along withgreatreliefandhope,biologicagentshavebroughttheneedforexpert vigilance requiring judicious selection of candidates,close observation, and careful attention to patient education.Theearlyyearsofbiologicalagentstherapyhasbeenassociatedwith the development of serious life-threatening infections inaddition to other well documented hematologic, immunologic,cardiovascular, and malignant adverse effects. Furthercomplications include the concomitant use of otherimmunosuppressive therapies, such as prednisone andmethotrexate, coexistent morbidities (Hyrich et al., 2006) andunderlying immune dysfunction associated with autoimmunediseases (Askling et al., 2005).Biologic therapies encompass agents with diverse modesof action. They include: (a) native biologic preparations; (b)recombinant peptides or proteins; (c) antibody-based therapy;(d) nucleic acid based therapies; and (e) cell and genetherapy (Sands, 1997; Bosani et al., 2009). At this time, thebiotechnology therapies that are being used in clinical practiceor investigated for the treatment of IBD are predominantlyrecombinant human proteins with immunoregulatory effects,monoclonalantibodies (chimeric, humanized, andfullyhuman)