Background
The incidence of fungal infections is steadily increasing in the last years due to invasive medical diagnosis and immunosuppressive treatment modalities. Despite development of new classes of antifungal agents [
1], the invasive fungal infections remain associated with unacceptable high mortality rates and represent a major cause of death worldwide [
2‐
7]. The emergence of significant resistance to the currently available antifungal therapies emphasizes the need for novel approaches to treat invasive fungal infections [
8,
9]. Invasive fungal infection are most commonly observed in individuals with immune defects or a compromised immune system, and the number of these patients is steadily increasing [
10]. Therefore, adjunctive immunotherapy to improve host defence is an attractive strategy to improve the outcome of patients with disseminated fungal infections.
In the past decade, major progress in the understanding of anti-fungal host responses has enabled the development of a number of novel molecular and cell-based immunotherapeutic approaches for invasive fungal infections [
11]. Although invasive candidiasis and aspergillosis are rather different in their pathogenesis, the major protective host response against both fungi is the effective induction of Th1 and IFN-γ responses [
12‐
16]. The Th1 cytokine response activates effector phagocytic cells that kill the fungus [
17]. Interestingly, Th1 immunity against
A. fumigatus was demonstrated to be cross-protective against
C. albicans[
18].
Interferon-gamma (IFN-γ), the prototype Th1 cytokine, promotes Th1 differentiation and skews the immune response towards a protective Th1 phenotype [
19]. As such, it has been implicated as a treatment option in (invasive) fungal infections [
20,
21]. Moreover, limited evidence suggests that recombinant IFN-γ (rIFN-γ) has a beneficial effect on the outcome of fungal infections in patients with chronic granulomatous disease (CGD) [
22], HIV [
23‐
25], leukemia [
26,
27], and in patients receiving organ transplants [
28]. However, it has not been investigated whether rIFN-γ actually enhances the immune response in these patients to explain these beneficial clinical effects.
In this report we describe a series of patients with invasive Candida and/or Aspergillus infections in whom we investigated the effects of treatment with rIFN-γ on the host innate and adaptive immune responses.
Discussion and conclusions
While several small clinical trials illustrated the beneficial clinical effects of adjuvant treatment with IFN-y, the proposed immunostimulating effect of IFN-γ as the mechanism of action has not been investigated. In this case series we demonstrate for the first time that adjunctive immunotherapy with rIFN-γ improves the leukocyte immune responses in patients with severe invasive fungal infections. This was primarily reflected by increased
ex-vivo pro-inflammatory cytokine responses of the innate immune system such as IL-1β or TNFα, as well as an increased production of the T-cell cytokines IL-17 and IL-22, which are known to play an important role in the anti-fungal host defence [
35,
41‐
45], and by an increase in HLA-DR expression in mHLA-DR expression in those patients with a low cellular expression as a measure of their immune suppression.
In addition to enhanced ex-vivo responses, subtle changes in the leukocyte differentiation were observed following IFN-γ treatment. Although there were no significant differences in total leukocyte numbers after treatment with rIFN-γ, shifts in leukocyte subpopulations such as increased monocyte and lymphocyte counts were apparent. While lymphocyte numbers increased after rIFN-γ therapy, it could not directly be attributed to a specific subset as all of them showed increased values. The most significant increase was that of CD8 cells one week after initiation of rIFN-γ therapy. Monocytes and lymphocytes are known to be crucial cells in the host defence against fungal infections. However, the increase of monocytes and lymphocytes during rIFN-γ therapy was accompanied by slightly decreased circulating granulocyte numbers. It is not known whether this reduction is due to activation and migration into the infected tissue, or whether a true decrease in granulocyte generation was induced by the treatment. Although the decrease in granulocyte numbers was slight, the fact that granulocytes, and especially neutrophils, are crucial in the antifungal host defence warrant careful monitoring of granulocyte numbers during IFN-γ treatment.
Several clinical studies and case reports have previously demonstrated beneficial effects of rIFN-γ in combination with antifungal therapy on outcome of fungal infections (for example in patients with CGD (n = 130) [
22,
46,
47], HIV (n = 173) [
23‐
25], leukaemia (n = 5) [
26,
27], and transplant patients (n = 7) [
28], in a patient with
S. aureus liver abscess and invasive
C. albicans infection [
48], in a patient with intracerebral aspergillosis [
49], in two patients with progressive chronic pulmonary aspergillosis [
50], and in two patients with idiopathic CD4 lymphopenia and cryptococcal meningitis [
51]). However, in contrast to our study,
ex-vivo immune responses in these patients were not investigated. Due to the limited number of patients and the very heterogeneous population, we could not assess clinical endpoints, although a mean mortality of 25% in the IFN-γ treated patients lies below the mean 40% estimated in patients with invasive fungal infections [
10,
52].
To the best of our knowledge, we are the first to describe mHLA-DR expression, a widely used marker of immunosuppression in (bacterial) sepsis patients [
53], in patients with invasive fungal infections. In all IFN-γ treated patients who showed baseline mHLA-DR levels below the immunoparalysis threshold of 50% and survived, IFN-γ mediated upregulation of mHLA-DR expression was observed. In agreement with the data presented in this case series, rIFN-γ has been shown to significantly increase numbers of HLA-DR-positive monocytes both in a human preclinical bacterial sepsis model and in septic patients [
31,
54]. Reduced production of TNFα by leukocytes
ex-vivo stimulated with LPS has also been shown to be marker of immunoparalysis in sepsis patients. In contrast to our study, mHLA-DR expression and
ex-vivo TNFα production were found to be highly correlated in bacterial sepsis patients [
54,
55]. A possible explanation for this discrepancy is that, in contrast with the emerging consensus that immunoparalysis renders patients more vulnerable to opportunistic infections in general [
53], different defects in immune defences may be responsible for enhanced susceptibility towards different pathogens.
Based on the apparent inverse correlation of baseline mHLA-DR levels with severity of underlying illness and tissue involvement, mHLA-DR levels seem to reflect disease severity and general immune status, and not specific immune defects per se. Hence, patients with invasive fungal infections and associated impaired anti-fungal immune responses will probably benefit more from immunostimulatory treatment compared to patients with only impaired physical barriers, e.g. due to indwelling catheters and apparent intact anti-fungal immune responses. Biomarkers reflecting the capacity of specific anti-fungal immune defences are required to identify patients who suffer from invasive fungal infections due to impaired cell-mediated immunity. It is important to identify such patients and attempt a tailored immunotherapeutic approach guided by the actual level and type of immunoparalysis of that specific patient. A blood based assay has been described that demonstrates a failure to induce IFN-γ expression in renal transplant patients and differences in IL-10 and TNFα expression [
56], which could be promising biomarkers to identify patients who could benefit from adjunctive immunotherapy.
The intracellular mechanism(s) through which the beneficial effects of IFN-γ are mediated remain to be elucidated. Recently it was proposed that IFN-γ exerts its effects at the transcription level [
57], while others have demonstrated that IFN-γ reverses tolerance-associated epigenetic modifications [
58]. Another possible mechanism involved in the IFN-γ mediated reversal of immunoparalysis is the downregulation of negative TLR regulators such as IRAK-M, a protein that negatively regulates LPS-induced inflammatory responses and contributes to the development of immunoparalysis [
59].
Administration of rIFN-γ was tolerated well. Several patients developed a mild fever upon administration, which responded well to acetaminophen treatment. No other side effects were observed. The most important limitation of the present study is the limited number of patients studied. Because the control group consisted of only three patients, no statistical analysis between the treatment and control groups could be performed. However, despite the small sample size, the increase in HLA-DR expression in patients with mHLA-DR expression levels below 50% and the increased
ex-vivo response of several cytokines that are crucial in antifungal host defence is a promising observation that underlines the potential of immunotherapy. The slow enrolment of patients presenting with candidemia was the main factor contributing to the decision to terminate the phase IIIb
Candida pilot-study early. With a reported incidence of 2.5-11 per 100,000 persons in Europe [
60], and based on previous epidemiological data in our hospital this low enrollment was not expected at the time of the initiation of the study. The much lower incidence of candidemia in the last two years in our hospital is most likely due to a new antibiotic stewardship introduced recently in our hospital, which has reduced the incidence of opportunistic infections. The cut-off value of mHLA-DR expression levels of 50% to distinguish between immunoparalyzed and immunocompetent patients is another limitation of this study, as this is an arbitrary value chosen. We chose this value because it is well below the 99% CI of mHLA-DR values in healthy volunteers [
31]. Therefore, patients with mHLA-DR below 50% do have an impaired antigen presenting capacity of their monocytes which we show to be enhanced by IFN-γ therapy. Whether this cut-off value truly represents immunoparalysis, reflected by enhanced susceptibility to secondary infections or reduced capacity to clear opportunistic infections, remains to be investigated. Furthermore, the use of a standardized analysis technique to quantify mHLA-DR, such as the Quantibrite method, is preferable, because it facilitates an objective comparison of mHLA-DR expression levels between studies and aids in the definitive establishment of a cut-off value to identify immunoparalyzed patients. Larger studies are required to confirm the data obtained here. To do so, multicentre studies should be facilitated in order to fully explore the potential of IFN-γ immunotherapy.
Our data indicate that adjunctive immunotherapy with rIFN-γ in patients with invasive fungal infections partially restores cell-mediated immunity. This suggests that IFN-γ treatment enhances anti-fungal immunity and larger studies are warranted to validate the findings reported here and to assess the impact of IFN-γ treatment on clinical outcome. Biomarkers of impaired anti-fungal immunity should be further investigated in order to identify patients who will benefit most from immunostimulatory therapy.
Competing interests
None of the authors have any competing interests regarding this study. The support for the immunological assessments in this study was provided by an unrestricted grant from BioMérieux.
Authors’ contributions
CD, PP, AP, BJK and MN conceived and designed the study. CD, JL and CBR screened and included patients. MG and FF carried out in-vitro experiments. CD, MG, JL, FV and MK analysed the data. MG, JL, FP. MK, PP, GM, AP and MN participated in the data interpretation. CD, MG, JL wrote the manuscript draft. MG, JL, MK, PP, BJK and MN contributed to writing the final manuscript. All authors read and approved the final manuscript.