The known herpesviruses resulting in human diseases include α- (HSV and VZV), β- (CMV, HHV-6,-7) and γ-herpesviruses (EBV and HHV-8). Herpesvirus infections are usually asymptomatic or subclinical in immunocompetent population. The virus becomes latent in infected cells after primary infection. When immune system is disordered or deficient, latent viruses may reactivate and result in symptomatic infections, even fatal complications [
24,
25]. The diagnostic methods of herpesvirus infections were summarized in Table
2.
Table 2
Laboratory diagnosis of herpesviruses
HSV | PCR(preferred); antigen detection [ 25] |
VZV | PCR(preferred); Immunofluorescent-antibody staining [ 25] |
CMV | PCR(preferred); CMV antigen (pp65) detection [ 24, 103] |
EBV | PCR; immunohistochemistry or in situ hybridization to detect EBV in biopsy specimens [ 25] |
HHV6-8 | |
HSV
Up to 80% of the healthy population have history of HSV-1 and −2 infections [
25]. HSV-1 infection is more common than HSV-2. After primary infection, the virus becomes latent in the neuronal cells. Historically, 80% of HSV-seropositive patient developed reactivation after allo-HSCT without antiviral prophylaxis [
104]. Fortunately, the incidence of HSV reactivation has now decreased to 0-3% because of prophylactic acyclovir [
56,
105]. HSV causes a spectrum of diseases, such as herpes, oesophagitis, bone marrow suppression, respiratory tract diseases, hepatitis and encephalitis in the recipients of allo-HSCT [
25]. Due to the high rate of HSV reactivation, prophylactic oral acyclovir has been administered routinely in allo-HSCT recipients. Intravenous acyclovir should be considered for patients with poor drug absorption [
17,
106]. Valacyclovir is an alternative prophylactic agent with good bioavailability [
107‐
109]. Acyclovir is recommended as the therapy for severe mucocutaneous or visceral HSV disease in transplant recipients [
25]. Valaciclovir and famciclovir are considered as alternatives for less serious manifestations of HSV diseases [
25]. The recommended drug for acyclovir-resistant HSV is foscarnet [
17]. Cidofovir might be effective to treat HSV infection which is resistant to both acyclovir and foscarnet [
110].
VZV
Varicella caused by primary VZV infection is a common childhood disease. After primary infection, VZV establishes latency in the dorsal root ganglia in immunocompetent host. The reactivation of VZV results in herpes zoster [
111,
112]. In the recipients of allo-HSCT, VZV is also an important cause of viral encephalitis. VZV immunization is advocated in recipients without a history of varicella. Varicella vaccine has been showed to be safe in children with leukemia, but few data are available in transplant recipients [
106]. Besides, vaccination of VZV-seronegative individuals who may be in contact with the patients during transplantation should be done [
25]. Zoster immune globulin (ZIG) and varicella-zoster immune globulin (VZIG) are passive antibody prophylaxis in seronegative recipients after exposure to varicella [
113]. Acyclovir and valacyclovir prophylaxis were proven effective in several trials [
9,
114‐
117]. Antiviral therapy with acyclovir is recommended in the treatment of VZV infection [
25]. Acyclovir has been shown to reduce the progression and dissemination of VZV infection [
118,
119]. Treatment with brivudin or famiciclovir is effective in immunocompromised population [
120,
121]. Foscarnet and cidofovir are alternative agents against acyclovir-resistant VZV infection [
122].
CMV
CMV infects 70-80% of the healthy individuals and establishes latency in peripheral blood monocytes and tissue macrophages. Till now, CMV remains one of the most important viruses and causes of death in the recipients of allo-HSCT. CMV-associated end-organ diseases include pneumonia, enteritis, hepatitis, retinitis and encephalitis, and so on; CMV syndrome is defined as CMV-associated fever without sign of CMV end-organ disease [
24]. Majority of CMV infections are caused by reactivation of virus which usually occurs within 3 months post-transplantation [
8,
24,
78]. Approximately 75% of CMV-seropositive recipients develop CMV reactivation, and 20-30% of these patients develop CMV disease without intervention [
17,
24]. Preemptive therapy based on CMV antigenemia or DNA-emia significantly reduces the development of CMV disease in allo-HSCT patients [
24,
123]. However, the mortality of CMV disease is more than 50% even with treatments [
67,
124]. The diagnosis of CMV infection includes CMV viremia, CMV syndrome and CMV end-organ disease [
24]. Historically, CMV antigen (pp65) detection was widely used in diagnosis of CMV infection. Recently, PCR is replacing antigenemia assay to be the preferred diagnostic method due to higher sensitivity [
24,
103].
Preemptive therapy based on CMV viremia has become the standard prevention of CMV diseases after transplantation [
24]. The first-line preemptive therapy is ganciclovir with a minimum duration of 2 weeks depending on whether CMV is detected at the end of the course [
24,
125,
126]. The main side effect of ganciclovir is bone marrow suppression which results in the increase of bacterial and fungal infection [
127,
128]. Valganciclovir is an alternative with good bioavailability [
129‐
131]. Foscarnet and cidofovir are the second-line prophylactic agents considering of drug-associated toxicity [
24].
Ganciclovir is the first-line treatment of CMV diseases. The recommended therapy of CMV pneumonia is a combination of intravenous ganciclovir and high dose immune globulin [
24,
124,
132]. In view of toxicity and effective rate, cidofovir and foscarnet are used as second-line therapy of CMV diseases [
24]. Ganciclovir resistance is uncommon and usually mediated through mutations in the UL97 gene. Cidofovir is used in the treatment of CMV disease which is resistant to ganciclovir and foscarnet, with an effective rate of 50% [
133]. Since it has been known that specific immune response to CMV is important to control reactivation, CMV-specific CTL has been used in prophylaxis and treatment of CMV viremia in several studies [
50,
51,
134]. Leen et al. reported that the response rate of CMV-specific CTL was 73.9% in the treatment of CMV diseases after allo-HSCT [
50].
EBV
Approximately 90% of healthy adults have been infected by EBV. After primary infection, EBV is latent in B cells (6, 8). Primary EBV infection or reactivation usually induces asymptomatic infection or infectious mononucleosis in immunocompetent people. However, EBV results in a spectrum of diseases in the recipients of allo-HSCT, ranging from fever, end-organ disease (pneumonia, encephalitis/myelitis, and hepatitis) to PTLD [
4]. Among these diseases, PTLD is most common [
4,
25]. Our data showed that the 3-year cumulative incidence of EBV disease were 15.6% in the recipients of allo-HSCT, with the PTLD incidence of 9.9% [
4]. EBV disease is usually caused by reactivation of latent virus after allo-HSCT. After transplantation, 14-65% of recipients developed EBV reactivation, depending on different risk factors that the recipients have [
37,
68,
135]. The diagnosis of EBV infection includes EBV viremia, probable end-organ disease and proven end-organ disease as well as PTLD [
25].
Since rapid increase of EBV-DNA loads in blood is considered to be related with subsequent EBV diseases [
136], routine monitoring of EBV viral loads is necessary after transplantation [
25]. Preemptive therapy based on EBV-DNA loads in blood and risk factors for EBV disease has yielded good results [
25]. Preemptive therapy is now developing mainly in two directions: adoptive cellular therapy (EBV-specific CTL) and B-cell depletion with monoclonal antibodies. EBV-specific CTL has been demonstrated effective to prevent EBV disease in several studies [
135,
137], but the production of CTL requires time. Besides, reduction of immunosuppressants is an ideal preemptive therapy, but frequently not available due to the risk of GVHD. Rituximab is easily available and has shown little toxicity [
138]. Based on the above, rituximab is recommended as the preferred preemptive therapy, followed by reduction of immunosuppresants and EBV-specific CTL in the European guidelines [
25].
The therapeutic strategies of EBV disease include anti-virus, restoration of T cell response and clearance of the EBV infected cells. Antiviral agents (e.g. acyclovir and ganciclovir) can reduce EBV replication, but is not active in PTLD presumably because that viral thymidine kinase expression is low during lytic phase and lack during latency [
139,
140]. Recently, a novel agent arginine butyrate, which induces EBV thymidine kinase transcription, has been shown
in vitro to render latently infected EBV-immortalized B cells susceptible to ganciclovir [
139,
140]. Treatments to restore T-cell reactivity include reduction of immunosuppressants and adoptive cellular therapy (CTL and DLI) [
25]. Anti-CD20 monoclonal antibody (rituximab) is used to clear EBV-infected B cells [
25].
According to the European guidelines, rituximab is a recommendation of highest priority for treatment of PTLD; other first-line treatment includes reducing immunosuppressants, EBV-CTL and DLI [
25]. Chemotherapy is recommended as the second-line therapy [
25]. The response rate of rituximab monotherapy was reported 44-69% whereas the relapse rate was 18-32% [
141‐
144]. Compared with rituximab, adoptive cellular immunotherapy has higher response rate (50-88%) and fewer relapse (0%) [
12,
145,
146]. Nevertheless, the utilization of adoptive cellular therapy is limited by the aforementioned disadvantages such as time and facilities required by CTL production as well as potential risk of GVHD caused by DLI [
12,
14,
97,
137,
145]. Chemotherapy is reported to induce remissions in 40-50% of the PTLD patients but with significant treatment-related mortality and relapse rate [
141,
147]. Therefore, in the ‘era of rituximab’, chemotherapy is barely used unless for CD20-negative PTLD or combination with rituximab [
145]. To date, there are no randomized trials to compare the efficacy between rituximab alone and rituximab combined with chemotherapy. Trappe et al. [
148] suggested that sequential first-line treatment with rituximab followed by chemotherapy is more efficacious than first-line rituximab monotherapy followed by chemotherapy at progression or relapse. In our study, we introduced a sequential therapeutic strategy that is rituximab-based treatments followed by adoptive cellular immunotherapy. The results revealed that this strategy might elevate response rate and decrease the relapse rate. Besides, this strategy might overcome the drawback of long time frame to product EBV-CTL and reduce the risk of GVHD caused by DLI. It remains a matter of discussion that whether histology subtypes of PTLD affect the outcome of rituximab-based treatments [
148,
149]. The prognosis of PTLD with extranodal or multi-organ involvement is dismal compared with isolated nodal involvement [
34,
150]. Some studies suggested that intrathecal rituximab is efficacious against PTLD with CNS involvement [
151‐
153]. Other therapeutic options include local radiotherapy and operation, which is mainly for the patients with significant compression symptoms [
145].
Clinical data on EBV fever and end-organ diseases are quite limited. Rituximab seems efficacious to treat EBV fever, with a response rate of 100% [
4,
154]. The treatment strategies of EBV end-organ diseases are similar with PTLD. However, the efficacy of rituximab monotherapy in patients with end-organ diseases seemed poorer than those with PTLD [
4].
HHV6-8
Approximately 50% of recipients develop HHV-6 reactivation after transplantation [
28,
30]. HHV-6 diseases include encephalitis, interstitial lung disease and delayed engraftment [
24,
31]. Two small-sampled studies suggested that ganciclovir might be effective to prevent HHV-6 reactivation in allo-HSCT recipients [
155,
156]. However, no widely accepted prophylactic strategy is recommended considering of the drug toxicity and the low incidence of HHV-6 diseases [
24]. Both ganciclovir and foscarnet were reported to be effective against HHV-6 diseases [
157].
HHV-7 infection/reactivation is infrequent in the recipients of allo-HSCT, and little information about HHV-7 diseases is available. Therefore, prophylaxis and treatment of HHV-7 infection remain unclear [
24].
HHV-8 is recognized the cause of Kaposi’s sarcoma in human immunodeficiency virus (HIV) infected patients [
158]. HHV-8 infection is rare in the recipients of allo-HSCT, and usually results in non-malignant diseases such as hepatitis, bone marrow suppression [
159,
160]. Currently, clinical data on prevention and treatment of HHV-8 diseases after allo-HSCT are based on case reports. Cessation of immunosuppressants and foscarnet were used for treatment, and the efficacy requires further study [
161,
162].