Transferred Herpes Simplex Virus Immunity after Stem-Cell Transplantation: Clinical Implications

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1 MAJOR ARTICLE Transferred Herpes Simplex Virus Immunity after Stem-Cell Transplantation: Clinical Implications W. Garrett Nichols, 1,3 Michael Boeckh, 1,3 Rachel A. Carter, 1,2 Anna Wald, 3,4,5 and Lawrence Corey 1,3,5 1 Program in Infectious Diseases and 2 Clinical Statistics, Fred Hutchinson Cancer Research Center, and Departments of 3 Medicine, 4 Epidemiology, and 5 Laboratory Medicine, University of Washington, Seattle Herpes simplex virus (HSV) commonly reactivates after stem-cell transplantation (SCT), despite acyclovir prophylaxis. Whether HSV-seropositive recipients with HSV-seronegative or type-discordant donors had more frequent and severe HSV infections than those with HSV type concordant donors was explored. Banked serum samples from HSV-positive SCT recipients and their donors were tested for the presence of HSV antibodies. HSV-1 positive SCT recipients from HSV-1 negative donors had more frequent and longer episodes than HSV-1 positive SCT recipients from HSV-1 positive donors; the proportion of patients receiving antiviral treatment for 110% of follow-up days was 27.4% versus 7.2% ( P!.001). Both HSV-1 visceral infection (9.8% vs. 2.2%; P p.001) and acyclovir resistance (5.8% vs. 1.8%; P p.03) were more common in type-discordant than -concordant patients, respectively; these associations were confirmed in multivariable models. Serological testing of donors can identify patients who are at highest risk for HSV-related morbidity, for whom prolonged prophylaxis or donor vaccination (once available) could be considered. In the absence of antiviral prophylaxis, reactivation of latent herpes simplex virus (HSV) occurs in 170% of HSV-seropositive stem-cell transplant (SCT) recipients during periods of chemotherapy-induced neutropenia and mucositis, resulting in painful ulcerative lesions of the oropharynx and perineum [1]. As a consequence of impaired immune responses in these individuals, herpes lesions exhibit prolonged viral shedding, heal Received 13 September 2002; revised 4 November 2002; electronically published 12 February Presented in part: 41st annual Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, December 2001 (abstract H-1112). The study was approved by the Institutional Review Board at the Fred Hutchinson Cancer Research Center. Financial support: National Institutes of Health (grants K23 AI to W.G.N., CA to L.C. and M.B., CA to L.C., and P01 AI to L.C. and A.W.); Infectious Disease Society of America Fellowship Award (to W.G.N.). Reprints or correspondence: Dr. W. Garrett Nichols, Program in Infectious Diseases, Fred Hutchinson Cancer Research Center, 1100 Fairview Ave. N, D3-100, Seattle, WA (gnichols@fhcrc.org). The Journal of Infectious Diseases 2003; 187: by the Infectious Diseases Society of America. All rights reserved / $15.00 more slowly, and have the potential to disseminate [2]. Thus, short courses of acyclovir prophylaxis are now commonly administered during the initial posttransplant period. This approach has dramatically reduced the incidence of early symptomatic HSV infection to!5% in the first month after transplantation [3 5]. Because immunity to HSV does not recover with neutrophil engraftment, reactivation of HSV occurs in up to 60% of seropositive patients after prophylaxis is stopped [1, 3, 5]. In addition, HSV recurrences may progress beyond the oral or genital mucosa. Indeed, severe pulmonary [6] and gastroesophageal [7] invasion have been well described and appear to be related to distal spread in the setting of poor local containment [6]. Although prolonged courses of acyclovir are usually effective in treating these events, acyclovir-resistant HSV infections have also become increasingly recognized after SCT [8 10]. The increasing use of unrelated [8, 9] and haploidentical [10] stem-cell donors has been associated with the development of acyclovir resistance, which suggests that the level of immunosuppression and delayed immune reconstitution are implicated. Transferred HSV Immunity JID 2003:187 (1 March) 801

2 The immune status of the stem-cell donor may be important for infections in the allogeneic SCT recipient, such as patients with chronic hepatitis B virus disease [11]. The impact of transferred donor immunity on the occurrence of HSV reactivation, visceral dissemination, or the development of acyclovir resistance within the SCT recipient are unknown. We explored the hypothesis that HSV-seropositive recipients whose donor was HSV-seronegative or type-discordant would have more frequent and more-severe HSV infections than those who received transplants from HSV type concordant donors. PATIENTS AND METHODS Patients. All patients undergoing SCT at the Fred Hutchinson Cancer Research Center (FHCRC) are routinely screened for HSV antibodies by ELISA, to determine the need for prophylactic acyclovir therapy. Patients who were HSV positive and were undergoing their first allogeneic SCT between 1 January 1989 and 31 December 1998 were eligible for inclusion in this observational cohort. Cytomegalovirus (CMV) seropositive patients or CMV-seronegative recipients of stem cells from CMV-seropositive donors were excluded, to remove the potential for confounding by the receipt of ganciclovir therapy [9, 12]. Conditioning for SCT [13 15] and prophylaxis [16, 17] and treatment [16] of graft-versus-host disease (GVHD) were performed as described elsewhere. Prophylaxis and treatment of HSV infections. For HSV prophylaxis, all patients received acyclovir (250 mg/m 2 intravenously [iv] twice daily) from the start of conditioning until day 30 after transplantation or the resolution of mucositis, whichever occurred first. Thereafter, patients with episodes of visually confirmed lesions received oral acyclovir (400 mg 5 times daily) for 1 week or until clinical resolution of lesions, whichever occurred later. Patients who failed to respond clinically were treated with high-dose iv acyclovir (500 mg/m 2 every 8 h) while laboratory testing for acyclovir resistance was pending. Patients with confirmed acyclovir resistance received iv foscarnet (60 mg/kg every 12 h) until the resolution of their lesions. Virological testing. Swabs of clinical lesions were placed in transport medium and delivered directly to the virology laboratory. All biopsy and bronchoalveolar lavage (BAL) specimens from patients with clinical diagnoses of gastroenteritis or pneumonitis were submitted for histopathologic testing and virus culture. Viral isolation and direct fluorescent antibody (DFA) testing of clinical samples were performed as described elsewhere; isolates were confirmed and typed with monoclonal antibodies [18, 19]. Testing for acyclovir resistance was performed for lesions that were clinically refractory to acyclovir therapy; in general, lesional swabs were sent for culture and resistance testing after 2 weeks of unsuccessful acyclovir therapy. Testing for resistance was performed as described elsewhere [20]; isolates with an IC mg/ml were reported as resistant. Banked patient and donor serum samples were tested for the presence of antibodies to HSV-1 and HSV-2 using a type-specific Western blot, as described elsewhere [21]. Definitions of HSV disease. Mucocutaneous HSV reactivation (at either oropharyngeal or anogenital sites) was defined as the presence of lesions consistent with herpetic ulceration, combined with a lesional swab positive by DFA or viral culture for HSV. A single episode of clinical reactivation was defined from the date of DFA or culture positivity until the end of antiviral therapy; recurrence of lesions with virological confirmation that occurred within 5 days of acyclovir cessation was considered to represent a continuing episode. The proportion of days with active HSV infection was defined as the number of days that the patient received antiviral treatment (other than prophylaxis) divided by the number of days that the patient was alive during the first year after transplant. HSV pneumonitis was defined as histopathologic (i.e., positive immunohistochemistry) or culture documentation of HSV from BAL or lung biopsy specimens in the presence of new or changing pulmonary infiltrates. HSV gastroenteritis was defined as histopathologic or culture documentation of HSV from enteric biopsy specimens in the presence of symptoms. Data sources. Clinical, laboratory, pharmacological, pathologic, and virological data were extracted from a prospectively entered, integrated database. Information in the computerized database is complete for the first 100 days after transplantation, after which patients are discharged from the FHCRC system. Patient clinical records after discharge are maintained in a longterm follow-up database, which includes prospectively entered data from a 1-year follow-up examination and review, yearly questionnaires to local physicians and patients, and copies of hospital discharge summaries, death certificates, and autopsy reports. Data were censored at death, day of second transplant, or day 365 after transplant (whichever occurred first). Statistical analysis. Comparisons of proportions were performed using the x 2 or Fisher s exact test. Proportions displayed graphically are shown with error bars calculated as 95% confidence intervals (CIs), using the normal approximation. Cumulative incidence curves were generated for HSV-related outcomes, where death and/or second transplant were considered to be competing risks. Logistic regression was used to examine the association of various factors with the probability of HSV-related outcomes (as defined above) in univariate models and later in multivariable models. Variables were included in multivariable models if they were significantly associated with the probability of any HSV-related outcome. Variables were considered to be significant if the 95% profile likelihood CI excluded JID 2003:187 (1 March) Nichols et al.

3 RESULTS Serum samples from 441 consecutive HSV-seropositive, CMVseronegative recipients of stem cells from CMV-seronegative donors were tested by HSV Western blot. Of these, 436 were found by Western blot to have either HSV-1 or HSV-2 antibodies (or both). These patients formed the study cohort; patient characteristics are shown in table 1. Follow-up data were available for all patients until death, second transplant, or 1 year after transplant; 191 patients died and/or received a second transplant before day 365 after transplant (median, 83 days after transplant; range, days after transplant); these patients contributed a total of 21,288 days of follow-up time. The remaining 245 patients survived to 1 year after transplantation; these patients contributed 185,000 patient-days of follow-up. Donor and patient HSV serostatus. Banked serum samples from patients and donors were assessed for the presence of type-specific antibodies to HSV-1 and HSV-2; cross-tabulation of results are shown in table 2. For patients who were HSV-1 positive, we hypothesized that the receipt of stem cells from HSV-1 positive donors (but not necessarily from HSV- 2 positive donors) would protect against severe HSV-1 related infections. As shown in table 2, 56.3% of HSV-1 seropositive patients received stem cells from donors who were positive for antibodies to HSV-1 (with or without antibodies to HSV-2). These were deemed to be concordant pairs with regard to HSV-1 related outcomes. Almost 39% of these HSV-1 positive patients received stem cells from HSV-negative donors, and 5.1% received stem cells from donors who were HSV-2 positive but lacked antibodies against HSV-1; these latter groups were thus discordant pairs. Among the 74 HSV-2 positive SCT recipients, 10 received stem cells from a type-concordant donor; the remaining 64 patients were discordant pairs. Clinical episodes of HSV reactivation during the first year after transplant. We first assessed the occurrence of virologically confirmed reactivation of HSV-1 among the 396 patients who were HSV-1 seropositive (with or without HSV-2 antibodies) by Western blot testing. Overall, 61% of the HSV- 1 positive patients had no episodes of HSV-1 reactivation within the first year after transplant, 22% had 1 episode, 10% had 3 episodes, and 7% had 3 distinct episodes of HSV-1 reactivation. HSV reactivation occurred more frequently and earlier in recipients of stem cells from HSV-1 negative (e.g., type discordant) donors (figure 1A). Breakthrough disease on acyclovir prophylaxis occurred in 7 (4.0%) of 173 recipients of HSV-1 negative stem cells but only 3 (1.3%) of 223 recipients of type-concordant transplants ( P p.17). As shown in figure 2A, the proportion of patients with 2 reactivations was higher among those who received stem cells from HSV-negative donors (40/153 [26%]) than those who received stem cells from HSV-1 positive donors (23/223 [10%]) ( P!.001). Recipients of stem cells from HSV-2 positive donors had more-frequent Table 1. Characteristics of 436 herpes simplex virus seropositive, cytomegalovirus (CMV) seronegative recipients of allogeneic stem-cell transplant from CMV-seronegative donors at the Fred Hutchinson Cancer Research Center, Characteristic No. (%) of subjects Age, years!20 77 (17.7) (47.0) (35.3) Male 276 (63.3) Underlying disease Acute leukemia 155 (35.6) CML/MDS 216 (49.5) Hodgkin disease/nhl/multiple myeloma 44 (10.1) Other 21 (4.8) Transplant type Matched allogenic 189 (43.3.) Mismatched/unrelated allogeneic 247 (56.7) Preparative regimen a Including TBI 331 (76.8) Without TBI 100 (23.2) Stem-cell source Bone marrow 413 (94.7) Peripheral blood or cord blood 23 (5.3) Acute GVHD Grade (31.0) Grade (69.0) NOTE. CML, chronic myelogenous leukemia; GVHD, graft vs. host disease; MDS, myelodysplasia; NHL, non-hodgkin s lymphoma; TBI, total body irradiation. a Five patients had missing data. reactivation, compared with recipients of stem cells from HSV type concordant donors (proportion with 2 episodes, 4/20 [20%] vs. 22/223 [10%]), although this difference was not statistically significant ( P p.19). We next examined the occurrence of virologically confirmed HSV-2 reactivation among the 74 HSV-2 seropositive SCT recipients. As shown in figure 2B, the proportion of patients with 2 reactivations appeared to be higher among those who received stem cells from HSV-negative donors (6/32 [19%]) than those who received stem cells from HSV-2 positive donors (0/ 10) ( P p.31). In addition, recipients of stem cells from HSV- 2 negative donors (e.g., discordant pairs) appeared to have more-frequent reactivation, compared with recipients of stem cells from HSV type-concordant donors (proportion with 2 episodes, 10/64 vs. 0/10). This difference again did not reach statistical significance ( P p.34), probably because of the small number of patients with HSV-2 concordant donors. Proportion of days treated for HSV infection during year 1 after transplant. An assessment of the number of HSV Transferred HSV Immunity JID 2003:187 (1 March) 803

4 Table 2. Herpes simplex virus (HSV) serostatus of 436 stem cell transplant recipient/donor pairs, as determined by Western blot. Recipient HSV serostatus Donor HSV serostatus, no. (%) of subjects HSV negative HSV-1 positive HSV-2 positive HSV-1 and HSV-2 positive HSV-1 positive (n p 396) 153 (38.6) 202 (51.0) 20 (5.1) 21 (5.3) HSV-1 positive only 142 (32.6) 182 (41.7) 18 (4.1) 20 (4.6) HSV-1 and HSV-2 positive 11 (2.5) 20 (4.6) 2 (0.5) 1 (0.2) HSV-2 positive (n p 74) 32 (43.2) 32 (43.2) 6 (8.1) 4 (5.4) HSV-2 positive only 21 (4.8) 12 (2.8) 4 (0.9) 3 (0.7) HSV-1 and HSV-2 positive 11 (2.5) 20 (4.6) 2 (0.5) 1 (0.2) reactivations as an end point fails to account for the duration of the clinical episode. In addition, this end point does not account for the time of follow-up, such that patients who die early after transplantation do not have the opportunity for multiple reactivations. We thus analyzed the impact of transferred HSV immunity on the proportion of days that patients required treatment for HSV infection during the first year after transplant. Overall, 156 (39%) of the HSV-1 seropositive patients had at least 1 clinical HSV-1 reactivation requiring antiviral treatment; the median duration of treatment per episode was 10 days (range, days). The median proportion of days that patients were alive and required antiviral therapy for HSV reactivation during the first year after SCT was 8% (range, 1% 81%). As shown in figure 2A, patients who received stem cells from HSV-negative donors received more treatment for HSV-1 than those who received stem cells from HSV-1 positive donors (number with 110% of days receiving treatment, 42/ 153 [27%] vs. 16/223 [7%]; P!.001). Similarly, patients who received stem cells from donors who were HSV-2 positive received more therapy than HSV-1 concordant pairs (number with 110% of days receiving therapy, 8/20 [40%] vs. 16/223 [7%]; P!.001). Among the 74 HSV-2 seropositive SCT recipients, similar trends were seen (figure 2B). Of the patients who received stem cells from HSV-negative donors, 9 (28%) of 32 required therapy for 110% of days alive during the first year after transplantation, compared with 0 of 10 patients who received HSV-2 positive stem cells ( P p.09). Recipients of HSV-1 positive stem cells were not significantly different from their type-concordant counterparts ( P p.56). Visceral HSV disease. We next explored whether donor HSV serostatus affected the occurrence of HSV-1 pneumonitis or gastroenteritis among the 396 HSV-1 seropositive SCT recipients. Overall, 22 (5.6%) of 396 patients had HSV-1 pneumonia ( n p 9) or gastroesophageal ( n p 14) disease at a me- dian of 68 days after transplant (range, days); one had both events, separated by a period of therapy. Visceral involvement occurred in 16 (10.5%) of 153 recipients of HSV-negative stem cells, 1 (5%) of 20 recipients of HSV-2 positive stem cells, and 5 (2.2%) of 223 recipients of HSV-1 positive stem cells. Thus, the receipt of HSV type discordant stem cells was significantly associated with the development of end-organ HSV- 1 invasion ( P p.001). As shown in figure 1B, the occurrence of HSV-1 visceral disease was temporally associated with the Figure. 1. Time to first herpes simplex virus (HSV) 1 reactivation (A) and visceral HSV-1 disease (B) among 396 HSV-1 positive stem cell transplant recipients, according to donor serostatus. 804 JID 2003:187 (1 March) Nichols et al.

5 cessation of acyclovir prophylaxis and differed according to HSV donor serostatus. No cases of HSV-2 end-organ disease occurred among the 74 HSV-2 positive SCT recipients. Acyclovir-resistant HSV. We next assessed the influence of donor HSV serostatus on the occurrence of acyclovir-resistant HSV-1 among HSV-1 seropositive SCT recipients. Virologically confirmed acyclovir resistance was documented in 14 (3.5%) of 396 patients at a median of 73 days (range, days) after transplant. Acyclovir resistance occurred in 8 (5.2%) of 153 recipients of SCT from HSV-negative donors, 2 (10%) of 20 of recipients of SCT from HSV-2 positive donors, and 4 (1.8%) of 223 of patients who received HSV-1 positive stem cells. Among HSV-1 seropositive patients, the receipt of typediscordant stem cells was thus associated with a significant increase in the risk for acyclovir resistance ( P p.03). One case of acyclovir-resistant HSV-2 occurred among the 74 HSV- 2 positive SCT recipients; this individual received stem cells from an HSV-negative donor. HSV-1 related outcomes: multivariable analysis. Finally, we performed univariate and multivariable analysis for each of our clinical end points, to determine whether the relationship between donor HSV serostatus and HSV disease in the transplant recipient was attenuated by other factors. In univariate analysis, receipt of stem cells from an HSV-1 seronegative donor, receipt of mismatched or unrelated transplants, underlying disease, and grade 2 4 GVHD were associated with at least 1 HSV-related outcome of interest ( 2 HSV-1 reactivation episodes, treatment of reactivation for 110% of time of followup, visceral HSV-1 disease, and acyclovir resistance; data not shown). Patient age, sex, and the use of total body irradiation in the conditioning regimen were not associated with HSV- 1 related outcomes. Multivariable models were then created for each outcome, with donor serostatus, underlying disease, transplant type, and acute GVHD as predictors. After controlling for these factors, the association between HSV-1 donor serostatus and all HSV- 1 related outcomes remained (table 3). DISCUSSION Figure. 2. Clinical herpes simplex virus (HSV) reactivations and proportion of days on therapy for HSV reactivation during the first year after transplantation among HSV-positive stem-cell transplant recipients, according to donor HSV serostatus. A, HSV-1 related lesions among HSV- 1 positive recipients. B, HSV-2 related lesions among HSV-2 positive recipients. HSV infections remain a significant problem for allogeneic SCT recipients in the acyclovir era up to 60% of seropositive patients have virus reactivation once prophylaxis is discontinued [1, 3, 5]. Our study showed that the transfer of type-specific donor immunity to HSV is the most important factor in suppressing virus reactivation; this association was independent of transplant type, underlying disease, or the occurrence of GVHD. Furthermore, among patients whose HSV infection did reactivate, transferred immunity appeared to shorten the duration of clinical lesions and protect the SCT recipient from the development of visceral disease and acyclovir resistance. The strengths of our study included the size of our cohort (for whom HSV prophylaxis and treatment protocols have been stable for over a decade) and the accurate classification of donor and recipient HSV serostatus by the use of type-specific Western blot testing. CMV-seropositive patients were excluded, to allow the effects of transferred immunity to become apparent in the absence of anti-cmv therapy [9, 12]. Demographic, clinical, pharmacological and virological data were extracted from a prospectively entered, integrated database, and standardizeddefinitions were used. Given that the majority of patients in the cohort were recipients of myeloablative bone marrow transplantation, however, further studies will need to investigate the Transferred HSV Immunity JID 2003:187 (1 March) 805

6 Table 3. Predictors of herpes simplex virus (HSV) 1 related outcomes among 396 HSV-1 seropositive stem-cell transplant recipients in multivariable analysis. Risk factor 12 reactivations Outcome, OR (95% CI) 110% of days alive on treatment Visceral HSV disease Acyclovir resistance Donor HSV-1 negative 3.1 ( ) 5.3 ( ) 4.7 ( ) 3.5 ( ) MM/URD transplant 1.8 ( ) 1.3 ( ) 0.7 ( ) 0.6 ( ) Underlying disease a Acute leukemia 0.5 ( ) 1.2 ( ) 0.7 ( ) 2.7 ( ) HD/NHL/MM 0.4 ( ) 0.8 ( ) 1.6 ( ) 1.0 ( ) Other diagnosis 0.2 ( ) 0.3 ( ) 1.1 ( ) 3.1 ( ) Donor HSV-1 negative 3.2 ( ) 5.6 ( ) 4.6 ( ) 3.3 ( ) Acute GVHD, grade ( ) 5.3 ( ) (4.7 ) b 2.3 ( ) Underlying disease a Acute leukemia 0.4 ( ) 1.1 ( ) 0.7 ( ) 2.9 ( ) HD/NHL/MM 0.3 ( ) 0.7 ( ) 1.7 ( ) 1.1 ( ) Other diagnosis 0.2 ( ) 0.3 ( ) 1.0 ( ) 3.1 ( ) NOTE. Separate models were constructed to account for the impact of MM/URD transplants and acute GVHD, because these variables were highly correlated. CI, confidence interval; GVHD graft vs. host disease; HD/NHL/MM, Hodgkin s disease, non-hodgkin s lymphoma, or multiple myeloma; MM/URD, mismatched or unrelated; OR, odds ratio. a Referent category: chronic myelogenous leukemia/myelodysplasia. b OR could not be quantified, because all patients with visceral HSV disease had acute GVHD. effect of transferred donor immunity among recipients of peripheral blood stem-cell or nonmyeloablative transplants. Transferred donor immunity has been well documented in the case of hepatitis B virus, where chronic surface antigen positive patients who received stem cells from surface antibody positive donors cleared their infections [11]. In the case of HSV, however, previous studies conducted by Meyers et al. [22] have shown that the presence of antibody to HSV in the stem-cell donor did not affect the reconstitution of lymphocyte responses to HSV antigen or the occurrence of HSV infection in the SCT recipient. That study, however, did not use typespecific HSV antibodies to assess serostatus and did not distinguish between viral shedding and clinical disease. Perhaps most important, the Meyers et al. [22] study was performed prior to the use of acyclovir as either prophylaxis or treatment; accordingly, 82% of seropositive patients shed HSV, a much higher proportion than the 40% with clinical disease in our acyclovir-treated cohort. Wade et al. [23] subsequently showed that the use of acyclovir alters the course of HSV infection after SCT patients treated with acyclovir had more frequent second episodes of HSV infection and lower subsequent lymphocyte responses to HSV than those not treated. It is likely that limiting antigenic exposure to HSV (by acyclovir prophylaxis or treatment) early after transplantation blunts the reconstitution of immunity against the virus [23]; the present study demonstrated that this phenomenon is most operative in the absence of transferred type-specific donor immunity. Changes in transplantation techniques over the past decade have resulted in a more profound and prolonged immunocompromised state. As a result, the incidence of HSV infections that are resistant to antivirals such as acyclovir appears to be rising [8 10, 24]. In previous studies, the receipt of stem cells from unrelated donors [8, 9] and the presence of GVHD [9] were associated with the development of acyclovir resistance; these factors were confirmed in our study. However, after controlling for these factors, the receipt of stem cells from a typediscordant donor was still associated with the development of virologically confirmed acyclovir resistance. We hypothesize that the reconstitution of T cell immunity (critical to the control of HSV reactivation) is impaired in these type-discordant donors; this leads to more frequent reactivations and a more frequent use of therapeutic acyclovir, which may select for resistance. Notably, extremely high rates of acyclovir-resistant HSV infection have been documented in recipients of T cell depleted SCT [8 10]; these patients are known to have profound and prolonged T helper cytopenia. Because no patients in our series received T cell depleted transplants, we were not able to assess the impact of donor HSV serostatus in this patient population. However, because donor CMV serostatus appears to be associated with CMV disease primarily in the T cell depleted setting [25], our findings likely apply to these patients as well. Of interest, the protection conferred by transferred donor immunity appeared to be type specific, such that HSV-1 positive SCT recipients who received stem cells from HSV-2 positive or HSV-negative donors had comparable burdens of HSV disease. This finding is best demonstrated by our analysis of 806 JID 2003:187 (1 March) Nichols et al.

7 the proportion of days requiring antiviral treatment for HSV reactivation during follow-up, because this end point is a reasonable surrogate for the number of days with active lesions (which could not be determined with absolute precision in the present retrospective study); this end point importantly incorporates the time to healing of lesions per episode in addition to the frequency of reactivation. The fact that type-specific immunity was not significantly associated with protection among HSV-2 positive patients likely reflects the rarity of these patients in our cohort. Our results are consistent, however, with other studies in the immunocompetent host, where previous infection with HSV-1 did not decrease incident infection with HSV-2 [26]. Given these results, type-specific antibody screening of both patients and donors would be necessary for the identification of the discordant pairs that could be targeted for alternate approaches. The use of type-specific HSV antibody testing for SCT donors and recipients could be applied in conjunction with several clinical algorithms. The preferential selection of HSV type-concordant donors is possible, but other priorities (such as HLA matching and matching for CMV serostatus) make this approach less applicable. The use of donor lymphocyte infusions (DLIs), which may be effective for other herpesvirus infections such as CMV [27], are not likely to be effective for type-discordant pairs. Indeed, the present study was conceived when DLI was considered for several patients with refractory HSV disease, but their stem-cell donors were found to be HSVseronegative. In the future, vaccination of HSV-seronegative donors could be performed prior to stem-cell collection to induce transferable immunity. Currently, prolonged prophylactic courses of acyclovir, valacyclovir, or famciclovir for type-discordant pairs seems the most logical approach. The use of prolonged prophylaxis would be expected to decrease both clinical and subclinical HSV shedding, as has been shown in the human immunodeficiency virus (HIV) positive host [28]; this would be expected to decrease clinical oropharyngeal disease and visceral dissemination. In addition, prophylaxis would theoretically decrease the overall probability that spontaneously occurring acyclovir-resistant mutants would emerge; indeed, famciclovir resistance was not seen in a trial of famciclovir for HSV suppression among HIV-infected patients [28]. For the most immunosuppressed patients, the dose, as well as the duration, of acyclovir prophylaxis is likely important, as is the case for CMV [29]. Langston et al. [10] noted that, in the setting of T cell depleted haploidentical transplantation, all HSVseropositive patients experienced clinical reactivation, and 5 of 7 had acyclovir-resistant infections when short courses of lowdose acyclovir (5 mg/kg every 8 h) were used for prophylaxis; both occurred infrequently when higher doses (10 mg/kg every 8 h) were used for longer periods. The optimal dose and duration for HSV type discordant pairs remains to be defined but likely depends on the cumulative level of immunosuppression. In summary, the absence of transferred type-specific donor immunity against HSV is associated with a significant increase in the burden of HSV disease among HSV-seropositive SCT recipients. These patients (who can be identified by type-specific HSV testing of donors) are likely to benefit from prolonged antiviral prophylaxis. The approach does add cost to donor and recipient screening and will also encounter the issue of uncovering unrecognized HSV-2 infection in some donors and SCT recipients (which may raise counseling issues). In addition, our center now uses routine acyclovir prophylaxis against varicella zoster virus (VZV) for 1 year among VZV-seropositive transplant recipients (which represent 90% of our patient population). As such, we have elected to treat all HSV-positive, VZV-negative SCT recipients with generic acyclovir (800 mg orally twice daily) until 100 days after transplantation. Should manipulation of donor immunity (e.g., via HSV vaccination) become feasible in the future, an investigation of this approach in lieu of prolonged prophylaxis would be considered. Acknowledgments We thank Chris Davis, Anne Cent, and the staffs of the Clinical Virology Laboratory and the Long-Term Follow-up Unit for their assistance in data collection. References 1. 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