Cryptococcosis in Solid Organ Transplant Recipients: Current State-of-the-Science

Abstract

Cryptococcosis remains a significant opportunistic infection in solid organ transplant recipients. Disease presentation and outcomes in the current era may be affected amongst other factors by the use of calcineurin-inhibitor immunosuppressive agents. It is being increasingly recognized that rapid reversal of immunosuppression in transplant recipients treated for cryptococcosis incurs the risk of immune reconstitution inflammatory syndrome that mimics worsening disease or relapse. This review summarizes the current state of knowledge regarding cryptococcosis in transplant recipients and highlights areas where future investigations are needed to further optimize outcomes in these patients.

Introduction

Invasive fungal infections (IFIs) are a significant complication in solid organ transplant (SOT) recipients (1-3). Improvements in transplantation practices, and wider employment of antifungal prophylaxis have led to a decline in the overall incidence of IFIs, particularly due to Candida and Aspergillus (3-5). The trends in the incidence of cryptococcosis in transplant recipients are less well delineated. However, it is plausible that major paradigm shifts in the approach to immunosuppression, for example increasing use of T-cell depleting antibodies may lead to more frequent occurrence of cryptococcosis in these patients (6). In addition, our ability to rapidly manipulate the immune system in SOT recipients receiving potent immunosuppressive agents has led to growing appreciation that complications such as Cryptococcus-associated immune reconstitution inflammatory syndrome (IRIS) that up until recently had only been consistently recognized in HIV-infected patients (7, 8) can also occur in SOT recipients (9, 10). Within the past few years, a series of reports have contributed to our evolving knowledge base of cryptococcosis in SOT recipients. The review herein summarizes the topical developments in the pathogenesis, epidemiologic characteristics, emerging syndromes, and management of cryptococcosis in SOT recipients.

Pathophysiology

Cryptococcal disease is generally considered to represent reactivation of quiescent infection although this has remained unproven (11, 12). In a study of SOT recipients, pre and post-transplant sera were tested for cryptococcal antibodies using an immuoblot assay (13). A majority of the patients who developed cryptococcosis exhibited serologic evidence of cryptococcal infection before transplantation (13). In fact, these patients developed cryptococcosis significantly earlier after transplantation than patients without pre-existent cryptococcal antibodies (13) suggesting that indeed a substantial proportion of transplant-associated cryptococcosis results from reactivation infection. On the other hand, evidence based epidemiological investigations also suggest acquisition of primary infection following transplantation (14, 15). Furthermore, isolates from a pet cockatoo and a renal transplant recipient with cryptococcosis showed identical genotypic profile suggesting recent acquisition of the yeast (16). Finally, rare cases of transmission from donor allograft have been reported following kidney (17) or lung transplantation (2).

Epidemiology

Cryptococcosis is the third most commonly occurring IFI in SOT recipients. An estimated 20−60% of the cases of cryptococcosis in non-HIV infected patients in US (18) and 17.4% in France (19) occur in SOT recipients. Surveillance data from 1985−2007 from France have documented a stable proportion of SOT recipients among HIV uninfected patients with cryptococcosis (20).Cryptococcosis represented 8% of the IFIs in SOT recipients in the Transplant Associated Infection Surveillance Network database (21). The overall incidence of cryptococcosis in SOT recipients is ∼2.8% and ranges from 0.3−5% (22).

Calcineurin-inhibitors are the mainstay of immunosuppression in SOT recipients in the current era. These agents do not appear to influence the incidence, but may affect the extent of cryptococcal disease (22). On the other hand corticosteroids are, (18, 19, 23-25) associated with an increased risk of cryptococcosis in all non-HIV infected hosts however, the precise daily dose that confers a higher risk in SOT recipients remains unknown T-cell depleting antibodies such as alemtuzumab are increasingly employed as induction therapy or as treatment of rejection in SOT recipients (6). This agent causes profound and lasting depletion of CD4+T cells and was associated with a dose-dependent increase in the risk for cryptococcosis (6). The cumulative incidence of cryptococcosis was 0.3% in SOT recipients who did not receive alemtuzumab or antithymocyte globulin , 1.2% in those who received a single dose, and 3.5% in the patients who received >1 doses of these agents (p=0.04) (6). IFIs occurred more frequently in SOT recipients who received alemtuzumab as antirejection as opposed to induction therapy (26).

A male predominance has been shown amongst HIV-infected and uninfected patients with cryptococcosis (19, 27-29). Among SOT recipients with cryptococcosis in France, 70.1% were male, a distribution matching the 78% rate in SOT recipients with cryptococcosis in the US (22). The gender difference in the susceptibility to cryptococcosis can be explained by influence of Xid locus on the X chromosome in mice or altered immune responses (30). Cryptococcosis in SOT recipients is typically a late-occurring infection; the median time to onset was 16 to 21 months after transplantation in 3 studies (21, 22, 31). The time to onset was earlier for liver and lung compared to kidney transplant recipients and may be due to a higher intensity of immunosuppression in the former subgroups (22).

Unlike C.neoformans var grubii (serotype A) which has no particular geographic predilection (32) and which causes most infections in SOT recipients, C.neoformans var neoformans (serotype D) is prevalent in Northern Europe. In France, 18% of the cryptococcal isolates in SOT recipients and in HIV-infected patients are serotype D (20). Until recently C. gattii had been regarded as a tropical and subtropical fungus. Its ecological niche however, has expanded to temperate regions and acquisition of cases within US has been documented, including in SOT recipients (33). C.gattii disease in Vancouver Island and Pacific Northwest has been documented after an incubation period of ∼6 months (34).

Clinical manifestations

Between 53−72% of the cryptococcal disease in SOT recipients is disseminated or involves the CNS (18, 22, 31, 35). Overall, 61% of the SOT recipients in one report had disseminated disease, 54% had pulmonary and 8.1% had skin, soft-tissue or osteoarticular cryptococcosis (31). Patients receiving a calcineurin-inhibitor-based regimen were less likely to have disseminated disease and more likely to have cryptococcosis limited to the lungs (31). Anticryptococcal activity of these agents that target the fungal homologs of calcineurin (36) was considered to account for these findings (31). Clinical strains of C. neoformans strains in SOT recipients however, remain susceptible to calcineurin-inhibitor agents suggesting that breakthrough infection is primarily due to their immunosuppressive effect and not due to the selection of drug resistant strains (37).

Thirty-three to 39% of the SOT recipients with cryptococcosis may have fungemia (22, 31, 38). Patient with CNS disease in one report were more likely to be fungemic than those without CNS disease (39). Other studies have documented lower rates of fungemia; of 11 SOT recipients in the French Cryptococcal A/D study, 8 had meningoencephalitis, 3 had urinary tract infection, and none had fungemia (29).

Approximately 33% of the SOT recipients with cryptococcosis have disease limited to the lungs (31). Pulmonary cryptococcosis may be detected as an incidental finding in asymptomatic patients (40). These patients presented with cryptococcal disease later in the post-transplant period and were receiving lower maintenance dosages of prednisone than symptomatic patients (31). Pulmonary cryptococcosis manifesting as acute respiratory failure is associated with grave prognosis (41).

Cutaneous cryptococcosis can present with papular, nodular, or ulcerative lesions or as cellulitis (42, 43). While cutaneous lesions largely represent hematogenous dissemination, skin has also been identified as a portal of entry of Cryptococcus and potential source of subsequent disseminated disease in SOT recipients (15). Such cutaneous infections in France are often due to serotype D (44).

Diagnosis and neuroimaging findings

As in all immunocompromised patients, a complete evaluation including collection of large volume CSF (≥ 1 ml or 20 drops), blood and urine analysis should be performed to delineate the extent of disease and to determine optimal treatment (29). CSF characteristics typically documented in SOT recipients with cryptococcal meningitis are outlined in Table 1.

Table 1.

Clinical and laboratory characteristics in solid organ transplant recipients with central nervous system cryptococcosis

Variable	Wu et al38	Husain et al26	Singh et al34
Number of patients	28	172	61
Cerebrospinal fluid characteristics
Opening pressure*(cm H20)	--	33 (14−70)	27(9−36)
WBC*(mm3)	188 (0−1,464)	33 (0−485)	78(0−1,200)
Glucose* (mg/dL)	52 (20−62)	36 (4−113)	47(2−181)
Protein*(mg/dL)	226 (69−1,015)	74 (16−715)	83(26−559)
Positive antigen**	100% (1:2,238)	100% (1:256)	98%(1:64)
Positive India ink	50%	80%	69%
Positive culture	77%	93%	86%
Serum cryptococcal antigen**	91% (1:3,048)	88% (1:128)	98%(1:512)
Fungemia	39%	33%	36%
Mortality	50%	40%	20%

^*Numbers represent mean or median (range).

^**Values represent the number of patients with positive antigen and mean/median antigen level.

Positive serum cryptococcal antigen has been reported in 88−91% of the SOT recipients with cryptococcal meningitis (22, 38). However, the serum and CSF antigen titers are generally lower in non-HIV infected hosts, including SOT recipients than in HIV-infected patients with CNS cryptococcosis (45). On the other hand in SOT recipients with CNS cryptococcal lesions, CSF antigen titers were higher in patients with leptomeningeal as opposed to parenchymal lesions, and hydrocephalus (p=0.015) (46). In contrast with studies primarily in HIV-infected patients (29, 47), high serum or CSF antigen titers did not correlate with mortality at 90 days or CSF sterilization at 2 weeks in SOT recipients (39)

Up to 33% of patients with CNS cryptococcosis may have CNS parenchymal lesions due to Cryptococcus (35, 39). Abnormal brain imaging has been identified as a poor prognostic factor in a population comprising HIV-infected and uninfected patients (29). Magnetic resonance imaging was more sensitive than computed tomography for the evaluation of CNS cryptococcosis in HIV-infected patients (48). Up to 7% of the pulmonary nodules in SOT recipients in certain parts of the world may be due to cryptococcosis (49, 50). Serum antigen positivity was documented in 83% of the SOT recipients with pulmonary cryptococcosis (46). Patients with concomitant extrapulmonary disease were more likely to have a positive antigen, and had higher antigen titers (46). Up to 38% of the SOT recipients may have pulmonary cryptococcosis detected as an incidental finding on imaging studies (46). Nodular densities were more likely than pleural effusions and infiltrates to present as incidentally detected pulmonary cryptococcosis (46).

Treatment principles and practices

There are no randomized, prospective trials of antifungal treatments devoted to cryptococcosis in SOT recipients. However, recommendations can be extrapolated from clinical trials in other hosts (51-54). It is important to separate treatment into those with disseminated cryptococcosis from those with extrameningeal non-disseminated, and mild to asymptomatic disease. For the management of meningoencephalitis or disseminated cryptococcosis, the 2008 IDSA Guidelines made explicit recommendations in transplant recipients (55). For induction therapy, the recommendation is AmBisome 4mg/kg/d or Abelcet 5mg/kg/d plus flucytosine 100mg/kg/d for 14 days followed by a consolidation phase with fluconazole 400−800 mg/d for 8 weeks, and finally maintenance or suppression phase using fluconazole 200−400mg/d for at least 6 months provided the immunosuppression is not augmented (Table 2).

Table 2.

Management of cryptococcal disease in solid organ transplant recipients

Meningoencephalitis
Induction	Duration
Preferred therapy
Ambisome 4 mg/kg/d or Abelcet 5 mg/kg/d plus 5 flucytosine 100 mg/kg/d in the absence of renal insufficiency	2 weeks
Alternative therapy
Ambisome 4 mg/kg/d or Abelcet 5 mg/kg/d	4 weeks
Consolidation
Fluconazole 400−800 mg/d	8 weeks
Maintenance
Fluconazole 200 mg/d	6−12 months
Isolated pulmonary cryptococcosis*
Fluconazole 400 mg/d	6−12 months

^*Disseminated disease must be excluded in all patients. Those with disseminated disease, diffuse pulmonary infiltrates, and acute respiratory failure should be treated with the same treatment regimen as cryptococcal meningoencephalitis.

Several important issues are addressed. Fungicidal therapy with a polyene and flucytosine is highly recommended since transplant recipients can possess a high burden of yeasts on initial presentation. Lack of flucytosine therapy during induction therapy has been shown to be an independent risk factor for mycological failure at week 2 (29). Furthermore, <14 days of flucytosine was associated with an increased risk of treatment failure at 90 days (20). The use of the lipid formulations of amphotericin B is favored over amphotericin B deoxycholate (≥ 0.7mg/kg/d) since many transplant recipients receiving calcineurin inhibitors already have comprised renal function and any further worsening of organ dysfunction should be avoided. Additionally, exacerbation of renal dysfunction may predispose to flucytosine toxicity; therefore maintenance of drug levels (2 hour post dose level of 30−80 ug/ml) and monitoring for bone marrow suppression is recommended.

Since relapse rate after 6 months of maintenance therapy is minimal, recommendations support a 6−12 months of maintenance therapy (55, 56). Drug interactions with fluconazole should be monitored however, long-term fluconazole is a relatively safe drug for use in SOT recipients. It is unlikely that any added benefit with other extended-spectrum azoles such as itraconazole, voriconazole, or posaconazole offer benefit (57, 58). In fact, in HIV-infected patients, itraconazole was inferior to fluconazole during consolidation/clearance phase (51).

For the treatment of pulmonary cryptococcosis, the IDSA Guidelines recommend fluconazole 400mg/d for mild-to-moderate disease in immunocompromised patients (55). Disseminated disease should be excluded and this requires a lumbar puncture, blood and urine cultures. Pulmonary cryptococcosis with or without serum antigen positivity can be treated similarly if disseminated disease is ruled out (31, 55). Positive cultures for C. neoformans from sterile and non-sterile body sites such as sputum warrant treatment even if the patient is asymptomatic. In lung transplant recipients, this yeast may colonize the donor organ and without treatment the infection will likely become invasive during immunosuppression and this requires early therapy (2).

Another issue specific to SOT recipients with cryptococcosis is the management of immunosuppression. The net immunosuppression should be reduced during therapy (9, 59), but precisely how this should be performed must be individualized one case at a time. Rapid reduction of immunosuppressives can have untoward side effects such as development of organ rejection and/or IRIS (10, 60). Thus, it is prudent to plan a gradual reduction while antifungal therapy is administered. The goal is eradication of infection but also preservation of allograft function. Although challenging at times, cryptococcosis can be successfully managed in a high proportion of SOT recipients. Other issues such as raised intracranial pressure, persistence, relapse, and alternative treatment strategies are well-covered in the IDSA Guidelines and pertain to all immunosuppressed hosts (55). Specifically, the use of interferon-γ as adjunct will need to be further scrutinized since it has the potential side effect of inducing graft rejection (31).

Routine antifungal prophylaxis for cryptococcosis is not recommended in SOT recipients since there are no evidence-based studies to support this and there is no precision in identifying specific high-risk group. At present, adequate treatment for primary cryptococcosis should obviate the need for secondary antifungal prophylaxis during treatment for any subsequent allograft rejection episodes. However, close clinical follow up will be necessary in this setting.

An important issue is timing of retransplantation in SOT recipients with graft failure after cryptococcosis. In kidney transplant recipients in whom the option of bridging dialysis exists, it is reasonable to consider retransplantation if the patient has received one year of antifungal therapy, has no signs or symptoms attributable to active cryptococcal disease, and has negative cultures at the site of original infection. In other SOT recipients where there is no bridging option, we recommend that the induction therapy should be completed, all positive culture sites should be rendered negative, cryptococcal antigen titer should be stable or ideally declining, and the patient should be on a stable regimen of fluconazole. In this case secondary prophylaxis with fluconazole after retransplantation should be employed for an arbitrary duration but one year post-transplant should be considered. We note that these recommendations are based largely on authors’ expert opinion as data for guidance on these issues is scant.

Immune reconstitution inflammatory syndrome

IRIS due to opportunistic pathogens has emerged as a major complication in patients with AIDS receiving HAART. Its incidence is estimated at 4 to 16/100 person-years in AIDS-associated cryptococcosis (61). IRIS has also been reported in SOT-related cryptococcosis with a prevalence of 4.8% (9). IRIS comprises a constellation of clinical manifestations due to an inflammatory tissue response in patients experiencing improvement in cellular immunity following reduction or cessation of immunosuppressive therapy (9).

Calcineurin-inhibitor agents and corticosteroids exert their immunosuppressive effect by preferentially inhibiting Th1 (IL-2 and IFN-γ) compared to Th2 (IL-10) responses (62, 63). Tacrolimus inhibits IFN-φ to a greater extent than cyclosporine (64, 65). Previous alemtuzumab therapy has also been recognized as a risk factor for IRIS (66, 67). The biologic basis of IRIS in SOT recipients is believed to be reversal of a Th2 to Th1 proinflammatory response upon withdrawal or reduction of immunosuppression. A potential role of Tregs and Th17 regulatory pathways in the pathogenesis of IRIS in SOT recipients has also been proposed (68).

In HIV-infected patients, fungemia, an extremely low CD4 cell count, cryptococcosis as an AIDS defining illness, lack of CSF sterilization at week 2, introduction of HAART within 1 to 2 months after the diagnosis of cryptococcosis and a rapid decline in HIV load after HAART have been recognized as risk factors for IRIS (8). In SOT recipients, IRIS occurred more frequently in patients receiving potent immunosuppressive regimens such as tacrolimus, mycophenolate mofetil and prednisone (9).

IRIS may either occur early (within few days) or late (up to several months) after the introduction of HAART (8). Time of onset of IRIS appears to be shorter in cases of IRIS involving the CNS. In SOT recipients, IRIS occurred a mean of 6 weeks after the initiation of antifungal therapy (9) and manifested as lymphadenitis, aseptic meningitis, cerebral abscesses, hydrocephalus or pulmonary nodules (9, 10). In kidney transplant patients, development of IRIS has been temporarily associated with allograft loss (60). IRIS needs to be distinguished from worsening cryptococcosis (although both entities can occur simultaneously) and from other opportunistic infections, or drug-related complications. There are no specific markers that can reliably establish the diagnosis of IRIS (48). Histopathologic examination often reveals granulomas containing macrophages with or without necrosis, a feature rarely observed at initial diagnosis of cryptococcosis in an immunosuppressed host (9).

There is no current definitive therapy for IRIS. Minor manifestations may resolve spontaneously within few weeks. Modifications in antifungal therapy are not warranted unless viable yeasts are documented in culture. Anti-inflammatory drugs such as corticosteroids in doses equivalent to 0.5 to 1 mg/kg of prednisone may be considered for major complications related to inflammation in the CNS and severe manifestations of pulmonary or other sites (10). The efficacy of thalidomide and other non-steroidal anti-inflammatory agents remains unproven.

Outcome

Mortality rates ranging in SOT recipients with cryptococcosis have typically ranged from 33−42% (22) and may be as high as 49% in those with CNS disease (22). Overall mortality in SOT recipients with cryptococcosis in the current era is ∼15% (31). With cryptococcosis limited to the lungs, mortality rate is as low as 2.8% (31). In a case series of 28 SOT recipients with cryptococcal meningitis, mortality correlated with altered mental status, absence of headache, and liver failure, the latter was an independent predictor for death (38). On the other hand, receipt of calcineurin-inhibitor agents was independently associated with a lower mortality, but renal failure at baseline with a higher mortality rate (31). Improved outcomes with the use of calcineurin-inhibitor agents may be attributable in part to their synergistic interactions with antifungal agents (48).

Summary

Cryptococcosis in SOT recipients is primarily comprised of disseminated disease or meningoencephalitis. Induction treatment in these patients should include a lipid formulation of amphotericin B preferably with flucytosine. Patients with non severe pulmonary cryptococcosis may be treated with fluconazole. Most cases of relapse occur in the first year of management supporting the use of suppressive therapy with fluconazole for 6−12 months. A rapid reduction of immunosuppression may be associated with IRIS that mimics worsening disease due to cryptococcosis. Characterization of risk factors for IRIS, discerning its immunologic basis, identifying diagnostic markers, and optimal management of IRIS in SOT recipients merits future investigations (69).