Immunocompromised patients constitute a heterogeneous population whose vulnerability to opportunistic viral infections is determined by the nature, depth, and duration of immune suppression rather than by any single diagnosis. Effective antiviral management in this population depends on accurate risk stratification, systematic virologic surveillance, and an understanding of when pre-emptive therapy prevents end-organ disease more safely than universal prophylaxis.
Determinants of Viral Risk. The immune deficits that predispose to opportunistic viral disease differ by underlying condition and by the specific arm of adaptive immunity most affected. Hematopoietic stem cell transplant (HSCT) recipients face the broadest viral vulnerability because engraftment requires ablation of both humoral and cellular immunity, with reconstitution taking months to years depending on graft source, conditioning intensity, and the presence of graft-versus-host disease (GVHD). Solid organ transplant (SOT) recipients receive targeted calcineurin inhibitor (CNI)-based immunosuppression that impairs T-cell activation particularly severely, predisposing to viruses controlled primarily by cytotoxic T lymphocytes (CTLs) — including cytomegalovirus (CMV), Epstein-Barr virus (EBV), adenovirus, and BK polyomavirus (BKPyV; BK virus). Human immunodeficiency virus (HIV) infection selectively depletes cluster of differentiation 4 (CD4)+ T-helper lymphocytes, with opportunistic viral complications becoming prevalent below CD4 counts of 50 to 200 cells per microliter depending on the specific pathogen.1
CMV Serostatus and the Donor/Recipient Risk Matrix. Cytomegalovirus risk in transplantation is stratified by the serostatus of donor and recipient, expressed as a donor-positive/recipient-negative (D+/R-) matrix. The D+/R- combination represents the highest-risk category because the recipient lacks pre-existing CMV-specific cellular memory and receives a graft containing latently infected donor cells that can reactivate under immunosuppression. Without antiviral prophylaxis, CMV infection occurs in 50% to 80% of D+/R- solid organ transplant recipients, with 20% to 30% developing CMV end-organ disease. The R+ category — any donor — carries 10% to 30% reactivation risk from recipient latent virus. The D-/R- combination carries the lowest risk, limited to primary acquisition from blood products or community exposure.2 HSCT recipients face an additional complexity: the graft-versus-host disease that frequently complicates allogeneic HSCT requires intensified immunosuppression that substantially amplifies CMV reactivation risk beyond what the initial serostatus matrix predicts.
Prophylaxis versus Pre-emptive Therapy. Two evidence-based strategies exist for preventing CMV end-organ disease in high-risk transplant recipients. Universal prophylaxis administers antiviral therapy to all high-risk patients for a defined post-transplant period regardless of virologic evidence of replication; the principal agents are valganciclovir (900 mg daily) in SOT and letermovir (480 mg daily, or 240 mg daily with cyclosporine) in CMV-seropositive HSCT recipients. Universal prophylaxis prevents early CMV disease effectively but is associated with late-onset CMV disease emerging after prophylaxis discontinuation, drug toxicity during the prophylaxis period, and, for ganciclovir-based agents, the myelosuppressive burden of prolonged treatment. Pre-emptive therapy monitors CMV viral load by quantitative polymerase chain reaction (PCR) at regular intervals and initiates antiviral treatment only when a defined viral load threshold is exceeded, before symptoms of end-organ disease develop. Pre-emptive therapy avoids unnecessary antiviral exposure but requires reliable laboratory infrastructure and patient adherence to surveillance schedules.3
Letermovir: Mechanism and Role in HSCT Prophylaxis. Letermovir is a first-in-class CMV-specific antiviral that inhibits the viral terminase complex, a multisubunit enzyme encoded by three CMV-specific genes (UL56, UL89, and UL51) that mediates packaging of CMV deoxyribonucleic acid (DNA) into procapsids during viral replication. Because letermovir targets a viral enzyme with no mammalian homologue and acts through a mechanism entirely distinct from viral DNA polymerase inhibition, it has no myelosuppressive activity — a major clinical advantage in HSCT recipients who are already at risk for prolonged neutropenia following conditioning. Letermovir is approved for prophylaxis in CMV-seropositive adult HSCT recipients from the day of transplant through day 100 post-transplant, and has demonstrated a significant reduction in clinically significant CMV infection compared with placebo in the pivotal phase 3 trial.4 Letermovir has no activity against herpes simplex virus (HSV) or varicella-zoster virus (VZV), and resistance — mediated by mutations in UL56 — emerges rapidly if letermovir is used as treatment for active CMV replication rather than prophylaxis in a low-replication setting.
D+/R-: highest risk (50-80% infection without prophylaxis); universal prophylaxis preferred. R+ (any donor): 10-30% reactivation; prophylaxis or pre-emptive monitoring. D-/R-: lowest risk; pre-emptive monitoring alone. HSCT R+: letermovir prophylaxis day 0 to day 100 (no myelosuppression). SOT D+/R-: valganciclovir 900 mg daily ×3-6 months; late-onset CMV risk after discontinuation.
Cytomegalovirus end-organ disease in immunocompromised hosts extends well beyond the retinitis that defined cytomegalovirus (CMV) as an acquired immune deficiency syndrome (AIDS)-defining illness in the pre-antiretroviral therapy era. In solid organ and hematopoietic stem cell transplant recipients, CMV most commonly manifests as colitis, pneumonitis, hepatitis, or encephalitis — conditions whose diagnosis frequently requires tissue sampling because blood CMV polymerase chain reaction (PCR) may be negative or low even in the setting of active tissue-invasive disease.
CMV Colitis: Diagnosis and Treatment. CMV colitis presents with profuse watery or bloody diarrhea, abdominal cramping, and fever in immunocompromised patients, most commonly in hematopoietic stem cell transplant (HSCT) recipients with graft-versus-host disease (GVHD) and in human immunodeficiency virus (HIV)-infected individuals with cluster of differentiation 4 (CD4)+ counts below 50 cells per microliter. The diagnosis requires colonoscopy with biopsy because blood CMV PCR is frequently negative or low-level in isolated colitis — the virus replicates locally within colonic mucosal cells without generating detectable systemic viremia in a proportion of cases. Histopathology demonstrates the pathognomonic owl-eye intranuclear inclusions of CMV-infected cells, with confirmatory CMV immunohistochemistry or in situ hybridization. Standard treatment is induction valganciclovir (900 mg twice daily) or intravenous (IV) ganciclovir (5 mg per kilogram every 12 hours) for 3 to 6 weeks, followed by secondary prophylaxis to prevent relapse in patients whose immune recovery remains incomplete.3 The distinction from GVHD colitis in HSCT recipients is clinically important because both conditions present similarly but require opposite management: GVHD requires intensified immunosuppression while CMV colitis requires antiviral therapy — tissue CMV diagnosis is essential before escalating corticosteroids.
CMV Pneumonitis: The Most Lethal Manifestation in HSCT. CMV pneumonitis is the most feared CMV complication in allogeneic HSCT recipients, carrying mortality rates of 30% to 50% despite antiviral therapy. It presents with progressive dyspnea, non-productive cough, hypoxemia, and bilateral interstitial infiltrates on computed tomography (CT) imaging. Diagnosis requires bronchoscopy with bronchoalveolar lavage (BAL); CMV detection in BAL fluid by PCR or culture, combined with compatible clinical and radiographic findings, establishes the diagnosis. As with CMV colitis, blood CMV PCR can be negative in pneumonitis. Treatment combines IV ganciclovir with intravenous immunoglobulin (IVIG) — the latter providing CMV-specific neutralizing antibodies that augment the antiviral response at the pulmonary mucosal surface. The role of IVIG is supported by observational data in HSCT recipients, where combination therapy consistently outperforms antiviral monotherapy; randomized evidence is limited by the rarity and severity of the condition.2 Reducing the intensity of immunosuppression where clinically feasible is an essential adjunct to pharmacological therapy.
CMV Encephalitis and Polyradiculopathy. CMV neurological disease occurs almost exclusively in profoundly immunocompromised patients, particularly those with advanced HIV infection and CD4 counts below 50 cells per microliter. CMV encephalitis presents with subacute cognitive decline, confusion, and cranial nerve palsies; magnetic resonance imaging (MRI) characteristically demonstrates periventricular signal abnormalities. CMV polyradiculopathy presents as ascending flaccid paralysis with sacral sensory loss and bladder dysfunction, mimicking Guillain-Barré syndrome but progressing more rapidly and occurring exclusively in severely immunocompromised patients. Cerebrospinal fluid (CSF) analysis shows a polymorphonuclear pleocytosis with hypoglycorrhachia in polyradiculopathy, and CMV PCR of CSF is highly sensitive for both conditions. Treatment requires IV ganciclovir, often with foscarnet added for additive antiviral effect at the central nervous system (CNS) level, given that CNS penetration of ganciclovir is relatively limited and disease progression despite monotherapy is frequent.5
Drug-Resistant CMV: Recognition and Management. CMV antiviral resistance should be suspected when quantitative CMV viral load fails to decline by at least one log10 copies per milliliter after two weeks of adequate ganciclovir therapy at therapeutic doses, or when CMV disease progresses despite treatment. Resistance is confirmed by genotypic testing of the UL97 (CMV phosphotransferase gene) and UL54 (CMV DNA polymerase gene) genes. UL97 mutations (most commonly at codons 460, 594, and 595, and the region around codon 520) confer ganciclovir resistance by impairing drug phosphorylation; these strains typically retain foscarnet and cidofovir susceptibility. UL54 mutations confer resistance at the level of the deoxyribonucleic acid (DNA) polymerase and may produce cross-resistance between ganciclovir, foscarnet, and cidofovir depending on the specific mutation; combined UL97 and UL54 mutations produce high-level multidrug resistance. Management of UL97-only resistance is to switch to foscarnet; UL54 mutations with foscarnet cross-resistance may respond to cidofovir; combined resistance is managed with combination foscarnet plus ganciclovir at reduced doses, reduction of immunosuppression, and consideration of investigational agents including maribavir, which targets the UL97 kinase through a binding site distinct from ganciclovir and retains activity against most UL97 resistance mutations.3
Blood CMV PCR can be negative in CMV colitis, pneumonitis, and encephalitis. A negative blood PCR does not exclude tissue-invasive CMV disease. Colonoscopy with biopsy (colitis), BAL (pneumonitis), and CSF PCR (encephalitis) are required for definitive diagnosis. Do not withhold antiviral therapy while awaiting tissue results in a deteriorating immunocompromised patient with compatible clinical presentation.
Epstein-Barr virus drives one of the most pharmacologically challenging complications of transplantation — post-transplant lymphoproliferative disorder (PTLD) — through a mechanism that is primarily immunological rather than replicative, rendering conventional antiviral agents largely ineffective and establishing immunotherapy as the cornerstone of treatment.
Epstein-Barr Virus (EBV) Biology and Latency in Immunocompetent Hosts. Epstein-Barr virus is a gammaherpesvirus that establishes lifelong latency in circulating cluster of differentiation 19 (CD19)+ B lymphocytes following primary infection, which is acquired by approximately 90% of adults worldwide through oropharyngeal secretion contact. In immunocompetent hosts, EBV-infected B cells expressing viral latency proteins are held under continuous surveillance by EBV-specific cytotoxic cluster of differentiation 8 (CD8)+ T lymphocytes (CTLs), which recognize and eliminate B cells expressing the viral nuclear antigens (EBNAs) and latent membrane proteins (LMPs) that drive B-cell proliferation. This CTL (cytotoxic T lymphocyte) surveillance maintains viral latency without allowing unchecked B-cell expansion. The clinical consequence of this biology is that EBV-driven lymphoproliferation occurs specifically when CTL surveillance fails — as it does under the T-cell-depleting immunosuppression required for transplantation.6
PTLD: Pathogenesis, Risk Factors, and Classification. Post-transplant lymphoproliferative disorder encompasses a spectrum of lymphoid proliferations ranging from polyclonal B-cell hyperplasia to monoclonal aggressive B-cell lymphomas histologically indistinguishable from diffuse large B-cell lymphoma (DLBCL). The pathogenesis involves EBV-driven transformation of donor or recipient B cells in the context of insufficient CTL surveillance. Risk factors for PTLD include EBV D+/R- serostatus (particularly in pediatric solid organ transplant (SOT) recipients, where primary EBV infection post-transplant carries very high PTLD risk), T-cell depletion of the graft, use of anti-thymocyte globulin (ATG) or anti-CD3 monoclonal antibodies (muromonab-CD3) for induction or rejection treatment, and high cumulative immunosuppressive burden. The incidence in SOT recipients ranges from 1% to 20% depending on organ type, with intestinal and multivisceral transplant recipients bearing the highest risk due to the large amount of donor lymphoid tissue transplanted.6
Antiviral Agents in PTLD: Mechanism and Limitations. Acyclovir, ganciclovir, and valganciclovir inhibit lytic EBV replication by blocking the viral deoxyribonucleic acid (DNA) polymerase after activation by EBV thymidine kinase (TK) — but the fundamental problem is that PTLD is driven by latently infected B cells expressing latency-phase gene products, not by cells undergoing active lytic replication. EBV TK is expressed only during lytic replication; latently infected B cells do not express TK and therefore cannot activate acyclovir or ganciclovir to their active triphosphate forms. Antiviral monotherapy with ganciclovir or acyclovir is therefore ineffective for established PTLD, though antivirals may have a role in preventing lytic EBV replication that seeds new B-cell infections in high-risk EBV-seronegative recipients receiving prophylaxis post-transplant.7
Rituximab and Immunosuppression Reduction. The two interventions with established efficacy in PTLD are reduction of immunosuppression (RIS) and rituximab, an anti-CD20 (cluster of differentiation 20) chimeric monoclonal antibody that depletes CD20-expressing B cells — the cellular reservoir of EBV in PTLD. RIS is the first intervention applied in all PTLD cases where clinically feasible, reducing or withdrawing calcineurin inhibitors and antimetabolites to allow reconstitution of EBV-specific CTL surveillance. Response rates to RIS alone are 20% to 40% in early polymorphic PTLD. Rituximab (375 mg per square meter intravenously weekly for four doses) produces response rates of approximately 40% to 60% in EBV-positive CD20-positive PTLD, with higher response rates in combination with RIS. Aggressive monomorphic PTLD histologically resembling DLBCL requires rituximab combined with chemotherapy (R-CHOP: rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone) following the principles of systemic lymphoma management, with careful attention to the competing immunosuppressive burden of both the chemotherapy and the underlying transplant regimen.7
Step 1: Reduce immunosuppression — always the first intervention; 20-40% response in early PTLD. Step 2: Rituximab weekly ×4 if no response or disease progression; 40-60% response. Step 3: R-CHOP chemotherapy for monomorphic PTLD resembling DLBCL. Antivirals (ganciclovir, acyclovir): not effective for established PTLD; EBV latency gene program does not express viral TK required for drug activation.
Adenovirus and BK polyomavirus (BK virus) represent two distinct viral threats in transplant recipients for which pharmacological options are limited, incompletely efficacious, and associated with significant toxicity. Both pathogens exploit T-cell immune deficiency to establish disseminated disease or end-organ injury, and both are managed primarily through immunosuppression reduction rather than antiviral therapy alone.
Adenovirus: Epidemiology and Clinical Spectrum. Adenoviruses are non-enveloped double-stranded DNA (dsDNA) viruses of the family Adenoviridae comprising more than 100 serotypes organized into seven species (A through G). In immunocompetent hosts, adenovirus causes self-limited respiratory, conjunctival, and gastrointestinal infections. In hematopoietic stem cell transplant (HSCT) recipients — particularly pediatric recipients of T-cell-depleted grafts or mismatched unrelated donor grafts — adenovirus can cause life-threatening disseminated disease with pneumonitis, hemorrhagic cystitis, hepatitis, and encephalitis. Disseminated adenovirus infection carries mortality rates exceeding 50% in severely immunocompromised HSCT recipients. Systematic adenovirus surveillance by peripheral blood polymerase chain reaction (PCR) is employed at many transplant centers for high-risk patients, enabling pre-emptive intervention before viremia progresses to end-organ disease.8
Cidofovir in Adenovirus: Mechanism, Efficacy, and Toxicity Protocol. Cidofovir is the principal pharmacological agent used for adenovirus disease in transplant recipients, though high-quality randomized evidence is absent and its use rests on observational data and mechanistic rationale. Cidofovir is an acyclic nucleoside phosphonate that undergoes cellular (not viral) enzyme-mediated phosphorylation to cidofovir diphosphate, which competitively inhibits viral DNA polymerases of a broad range of double-stranded DNA (dsDNA) viruses including adenovirus, CMV (cytomegalovirus), HSV (herpes simplex virus), VZV (varicella-zoster virus), BKPyV, and orthopoxviruses — with no requirement for viral thymidine kinase or UL97 (CMV phosphotransferase) activation. Standard dosing for adenovirus in transplant recipients is cidofovir 5 mg per kilogram intravenously weekly for two doses (induction), then every two weeks (maintenance), with mandatory probenecid administration (2 g orally three hours before infusion; 1 g at two and eight hours after) and 1 L of normal saline pre-infusion to reduce organic anion transporter 1 (OAT1)-mediated proximal tubular accumulation and nephrotoxicity. Despite the probenecid protocol, nephrotoxicity occurs in up to 20% to 65% of patients; renal function and proteinuria must be checked before each dose, and cidofovir is contraindicated when creatinine clearance (CrCl) falls below 55 mL per minute.8
Brincidofovir and Adoptive T-Cell Therapy. Brincidofovir (hexadecyloxypropyl-cidofovir, Tembexa) is a lipid-conjugate prodrug of cidofovir that enters cells via lipid endocytosis rather than through OAT1-mediated renal tubular uptake, avoiding the proximal tubular accumulation that causes cidofovir nephrotoxicity. Intracellular phospholipases cleave the lipid conjugate to release cidofovir diphosphate directly within the cell, achieving higher intracellular drug concentrations than systemically administered cidofovir at a fraction of the nephrotoxic dose. Brincidofovir was FDA-approved in 2021 for smallpox (orthopoxvirus) but has been investigated for adenovirus in HSCT recipients; a phase 3 trial in adenovirus-infected HSCT recipients did not demonstrate survival benefit over placebo, a result attributed in part to immunosuppressive effects of brincidofovir itself at the doses studied. Adoptive transfer of adenovirus-specific CTLs — expanded ex vivo from donor or third-party Epstein-Barr virus (EBV)-specific T-cell lines — represents an emerging cellular immunotherapy strategy with promising early results in adenovirus disease refractory to cidofovir.8
BK Polyomavirus Nephropathy: Pathogenesis and Management. BK polyomavirus is a ubiquitous human polyomavirus that establishes latency in renal tubular epithelial cells and uroepithelium following primary childhood infection. Under the intense T-cell suppression required to prevent kidney transplant rejection, BKPyV reactivates and replicates in tubular cells, producing a pathological picture of BK polyomavirus nephropathy (BKPyVAN) that threatens allograft survival. The clinical sequence begins with asymptomatic BKPyV viruria, progresses to BKPyV viremia (detectable by plasma PCR), and culminates in nephropathy with allograft dysfunction if untreated. Monitoring plasma BKPyV PCR monthly for the first six months and every three months through the second year of transplant allows pre-emptive immunosuppression reduction before nephropathy is established. The cornerstone of BKPyVAN management is reduction of immunosuppression — reducing or switching calcineurin inhibitor (CNI), reducing antimetabolite dose — which allows reconstitution of BKPyV-specific cytotoxic T lymphocyte (CTL) clearance. No antiviral agent is FDA-approved for BKPyV; cidofovir at low doses (0.25 to 1 mg per kilogram every one to two weeks without probenecid) has been used with modest observational benefit, and leflunomide (an immunomodulatory drug that also inhibits BKPyV replication through pyrimidine synthesis inhibition) has been used in refractory cases, though evidence for both is limited.9
Plasma BKPyV PCR monthly for months 1-6, then every 3 months through month 24. Threshold for action: plasma viral load >10,000 copies/mL (some centers use 1,000 copies/mL). Action: reduce immunosuppression (reduce CNI target trough, reduce or stop antimetabolite). No approved antiviral; low-dose cidofovir or leflunomide for refractory cases only. Allograft biopsy confirms BKPyVAN when plasma viral load is elevated and creatinine is rising.
Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system caused by lytic JC (John Cunningham) polyomavirus (JCPyV) infection of oligodendrocytes — the myelin-producing cells of the central nervous system (CNS) — in the setting of profound T-cell immune deficiency. PML carries a historically grave prognosis, and effective pharmacological treatment remains elusive; management centers on immune restoration rather than direct antiviral therapy.
JC Virus Biology and Latency. JC polyomavirus is acquired in childhood through oropharyngeal or urinary-fecal routes and establishes persistent latent infection in renal tubular epithelial cells, tonsillar stromal cells, and bone marrow-derived cells including B lymphocytes and circulating hematopoietic progenitor (CD34+) cells. Seroprevalence in adults reaches 50% to 80% worldwide, meaning the majority of immunocompromised patients harbor latent JCPyV before their immune deficiency develops. Under conditions of profound T-cell immune suppression, JCPyV undergoes rearrangement of the non-coding control region (NCCR) from the archetypal urinary strain configuration to a neurotropic rearranged form with enhanced transcriptional activity in oligodendrocytes, enabling productive lytic infection of CNS myelin-producing cells.10
Clinical Contexts and Natalizumab-Associated PML. PML was historically encountered almost exclusively in patients with advanced human immunodeficiency virus (HIV) infection (cluster of differentiation 4 (CD4)+ counts below 200 cells per microliter), hematological malignancies under chemotherapy, and organ transplant recipients. The modern epidemiology of PML expanded substantially with the recognition of natalizumab-associated PML — a complication of natalizumab (anti-alpha-4 integrin monoclonal antibody) therapy for multiple sclerosis and Crohn disease. Natalizumab prevents lymphocyte trafficking from blood into tissues including the CNS by blocking very late antigen-4 (VLA-4) binding to vascular cell adhesion molecule-1 (VCAM-1); this mechanism simultaneously reduces CNS inflammation beneficially in multiple sclerosis while preventing JCPyV-specific cytotoxic T lymphocyte (CTL) surveillance of the CNS compartment. PML risk with natalizumab is stratified by JCPyV antibody index (a quantitative measure of anti-JCPyV antibody titer), treatment duration beyond 24 months, and prior immunosuppressant use — a validated risk stratification model that guides decisions about natalizumab continuation or switching to safer alternative therapies.10
Diagnosis of PML. PML presents as subacute progressive focal neurological deficits corresponding to multifocal areas of white matter demyelination, most commonly affecting the parieto-occipital regions, frontal lobes, and cerebellar white matter. Magnetic resonance imaging (MRI) demonstrates characteristic T2 (T2-weighted imaging sequence) hyperintense lesions without mass effect or surrounding edema, typically sparing the cortex and subcortical U-fibers early in disease. Diagnosis is established by detection of JCPyV deoxyribonucleic acid (DNA) in cerebrospinal fluid (CSF) by quantitative polymerase chain reaction (PCR), which has sensitivity of approximately 70% to 90% and specificity exceeding 95% in symptomatic patients with characteristic MRI findings. Brain biopsy demonstrating the triad of demyelination, bizarre enlarged oligodendrocyte nuclei, and enlarged astrocytes confirms PML when CSF PCR is negative in a patient with high clinical suspicion. Biopsy tissue immunostaining with anti-SV40 antibody (which cross-reacts with JCPyV large T-antigen) provides pathological confirmation.10
Management: Immune Restoration as the Primary Strategy. No antiviral agent has demonstrated efficacy against JCPyV in randomized controlled trials, and cidofovir — the agent most extensively studied — failed to show benefit in multiple clinical series. Management therefore centers entirely on restoring JCPyV-specific CTL immunity. In HIV-associated PML, immediate initiation or optimization of antiretroviral therapy (ART) to suppress HIV viral load and allow CD4 count recovery is the most important intervention, producing stabilization or improvement in approximately 50% of cases. In natalizumab-associated PML, discontinuation of natalizumab is mandatory; plasma exchange (PLEX) accelerates natalizumab clearance and has been used to hasten immune reconstitution, though the rapid return of lymphocyte CNS trafficking after natalizumab withdrawal can precipitate a severe immune reconstitution inflammatory syndrome (IRIS) affecting the PML lesions. Checkpoint inhibitor therapy with pembrolizumab has been used investigationally to enhance JCPyV-specific T-cell responses in PML, with preliminary signals of benefit in small case series.10
Low risk (<1:10,000): JCPyV antibody negative AND duration <24 months. Intermediate risk: JCPyV antibody positive, index <0.9, AND duration <24 months. High risk (>1:100): JCPyV antibody index >1.5 AND duration >24 months AND prior immunosuppressant use. Annual MRI surveillance recommended for JCPyV antibody-positive patients. PML suspected: discontinue natalizumab immediately; CSF JCPyV PCR; consider PLEX; monitor for IRIS.
Human herpesvirus 6B (HHV-6B) occupies a uniquely complex niche in transplant medicine: it causes post-transplant encephalitis through a mechanism that is difficult to distinguish from drug toxicity or other infectious encephalitides, its deoxyribonucleic acid (DNA) can integrate into host chromosomes producing spuriously elevated polymerase chain reaction (PCR) results, and its treatment competes pharmacokinetically with the immunosuppressants required to maintain allograft function.
Human Herpesvirus 6 (HHV-6) Biology: Two Species and Chromosomal Integration. Human herpesvirus 6 exists as two distinct species — HHV-6A (Human Herpesvirus 6A) and HHV-6B — both betaherpesviruses that share approximately 90% genomic sequence identity but differ in tropism and clinical associations. HHV-6B causes exanthem subitum (roseola infantum) during primary infection in early childhood and subsequently establishes latency in cluster of differentiation 4 (CD4)+ T lymphocytes, monocytes, and macrophages. A biologically distinctive feature of HHV-6 (both species) is the capacity for chromosomal integration — the viral genome integrates into the telomeric regions of host chromosomes and is then transmitted with every cell division, including vertically to offspring. Inherited chromosomally integrated HHV-6 (iciHHV-6) is present in approximately 1% of the general population and produces constitutively high plasma HHV-6 DNA levels (typically exceeding 1 million copies per milliliter) that can be misinterpreted as active replication. Distinguishing iciHHV-6 from true active HHV-6 replication requires demonstrating HHV-6 DNA in hair follicle cells (where iciHHV-6 will be present) compared with plasma (where both iciHHV-6 and active viremia will be positive).11
HHV-6B Post-Transplant Encephalitis: Clinical Recognition. HHV-6B encephalitis is a recognized complication of allogeneic hematopoietic stem cell transplant (HSCT), typically occurring two to six weeks after transplant coinciding with the period of profound T-cell lymphopenia following conditioning. It presents with acute confusion, anterograde amnesia, seizures, and characteristic magnetic resonance imaging (MRI) findings of bilateral medial temporal lobe signal abnormality on T2-weighted (T2) and fluid-attenuated inversion recovery (FLAIR) sequences — a pattern resembling herpes simplex encephalitis on imaging but without the hemorrhagic component. Cerebrospinal fluid (CSF) HHV-6 PCR is required for diagnosis; plasma HHV-6 PCR is less specific in the transplant setting because low-level reactivation is extremely common (occurring in 30% to 70% of allogeneic HSCT recipients) without neurological disease. Treatment is IV ganciclovir (5 mg per kilogram every 12 hours) or foscarnet (90 mg per kilogram every 12 hours), continued for a minimum of three weeks or until clinical and virologic response is documented.11
Immune Reconstitution Inflammatory Syndrome in the Transplant Setting. Immune reconstitution inflammatory syndrome (IRIS) is a paradoxical worsening of existing or previously subclinical infections that occurs as immune function recovers following a period of profound immunosuppression. In human immunodeficiency virus (HIV)-infected patients initiating or re-initiating antiretroviral therapy (ART), IRIS most commonly manifests as exacerbation of mycobacterial, cytomegalovirus (CMV), or cryptococcal disease as expanding CD4+ T cells mount inflammatory responses against antigens. CMV IRIS characteristically presents as paradoxical worsening of CMV retinitis or uveitis despite effective virologic suppression, and requires careful coordination of CMV treatment, ART, and sometimes short-course corticosteroids to suppress the excessive inflammatory response without abrogating immune recovery. In the transplant setting, IRIS-like phenomena occur as immunosuppression is reduced to manage opportunistic infections — reduction of calcineurin inhibitor (CNI) doses that allows T-cell reconstitution can unmask previously subclinical Epstein-Barr virus (EBV), adenovirus, or HHV-6 infections simultaneously, creating complex polyviral management challenges that require close coordination between transplant and infectious disease teams.1
Polypharmacy and Drug Interactions in Transplant Antiviral Management. The management of opportunistic viral infections in transplant recipients is complicated by extensive pharmacokinetic interactions between antiviral agents and the immunosuppressants required to maintain allograft function. Valganciclovir and ganciclovir are renally eliminated; their myelosuppressive effects are additive with mycophenolate mofetil (MMF), which impairs lymphocyte and granulocyte proliferation through inosine monophosphate dehydrogenase (IMPDH) inhibition — requiring complete blood count (CBC) monitoring at least twice weekly and dose reduction or temporary MMF suspension during intensive ganciclovir induction. Cidofovir nephrotoxicity reduces renal clearance of CNIs (cyclosporine, tacrolimus), requiring more frequent CNI level monitoring during cidofovir therapy. Foscarnet chelates ionized calcium, magnesium, and potassium, and its electrolyte-depleting effects can precipitate tacrolimus-associated QTc prolongation if hypomagnesemia is not corrected; electrolyte replacement is mandatory before each foscarnet infusion. Letermovir is a moderate inhibitor of cytochrome P450 3A4 (CYP3A4) and cytochrome P450 2C8 (CYP2C8); it increases tacrolimus and cyclosporine plasma concentrations by 40% to 45% and 15%, respectively, requiring CNI dose reduction at letermovir initiation with frequent trough level monitoring during the first two to four weeks of combined therapy.3
Valganciclovir + mycophenolate: additive myelosuppression; CBC twice weekly; reduce or hold MMF if ANC <1000. Cidofovir: nephrotoxic; reduces CNI clearance; monitor CNI levels more frequently during therapy. Foscarnet: hypomagnesemia → QTc prolongation with tacrolimus; replace Mg before every infusion. Letermovir: CYP3A4 inhibitor; reduces tacrolimus and cyclosporine clearance by 40-45% and 15%; reduce CNI dose at letermovir start; check levels weekly for 4 weeks.
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