The association of interleukin‐36 staining intensity with histopathologic findings of eosinophil count and spongiosis in patients with psoriasis: A secondary analysis of a retrospective immunohistochemical and chart review pilot study

Interleukin-36 (IL-36) is a potent proinflammatory cytokine that is a central driver of psoriasis,1, 2 and recent studies have found that IL-36 immunohistochemical staining can serve as a diagnostic biomarker of psoriasis. While the classic histopathologic criteria for a diagnosis of psoriasis include the absence of eosinophils,3 IL-36, interestingly, also promotes eosinophilic inflammation in bullous pemphigoid4 and allergic rhinitis,5 among other conditions. We aimed to examine the association of IL-36 staining intensity and histopathologic findings of eosinophil count and spongiosis. To this end, we performed a secondary analysis of a retrospective study that included patients with psoriasis with biopsies that have been stained with hematoxylin and eosin (H&E) and IL-36. In brief, our original study was a retrospective, single institution, immunohistochemical and chart review pilot study at the University of California, San Francisco that included patients with biopsy-confirmed psoriasis who had been treated with biologic therapies from 2010 to 2020.6 We performed IL-36 staining for each case and IL-36 staining intensity in the upper epidermis was graded with the following numerical scores, as established in the literature7, 8: 0 negative, 1 focal weak, 2 diffuse weak, 3 focal strong, and 4 diffuse strong. Each H&E-stained biopsy specimen was also evaluated to confirm the diagnosis of psoriasis and determine the degree of spongiosis (mild: evidence of mild intercellular edema with subtle visible desmosomal attachments between spinous layer keratinocytes; moderate: increased evidence of intercellular edema with prominent stretching of desmosomal attachments but without intraepidermal vesicle formation; and severe: intercellular edema with intraepidermal vesicle or bulla formation), and quantify the maximum eosinophil count per high powered field (HPF; ×400 magnification) using the hotspot method of screening the entire biopsy for the highest number of eosinophils and counting one HPF from that area. All diagnoses, staining, and histopathologic features were confirmed and reviewed by a board-certified dermatopathologist (JPN). The study was approved by the Institutional Review Board (#17-21933) and was performed in accordance with the Declaration of Helsinki. For full details of the study design, immunohistochemistry, and methods, please see Zhang et al. (2023).6 We used the one-way analysis of variance to compare mean IL-36 scores by eosinophil count (grouped categorically as 0, 1–2, and ≥3 eosinophils per HPF, as previously studied in the literature9) and degree of spongiosis. Stratifying by low IL-36 (scores of 0–2) vs. high IL-36 (scores of 3–4) expression (these categorizations were selected based on prior literature and feasibility of clinical use), we also compared the mean eosinophil count in psoriasis biopsies using the t-test. Among the biopsies of the 75 patients with psoriasis, 66 (88%) demonstrated high IL-36 staining: 33 (44%) had a focal strong staining (IL-36 score of 3) and 33 (44%) had diffuse strong staining (IL-36 score of 4) (Table 1; Figure 1A). There were few patients with negative or weak IL-36 staining (12%, N = 9 with IL-36 scores of 0–2).6 Eosinophil counts in psoriasis biopsy specimens ranged from 0 to 22 per HPF, with 53.3% (N = 40) with no eosinophils, 25.3% (N = 19) with 1–2 eosinophils, and 21.3% (N = 16) with ≥3 eosinophils (Table 1; Figure 1B). Mild spongiosis was present in most biopsy specimens (N = 61, 81.3%), with fewer numbers that showed moderate (N = 13, 17.3%) and severe (N = 1, 1.3%) spongiosis (Table 1; Figure 1C). The mean IL-36 expression score did not significantly differ by eosinophil count (p = 0.17) or by degree of spongiosis (p = 0.88). When stratifying by low versus high IL-36 scores, there was no statistically significant difference in eosinophil counts, though there was a trend for biopsies with high IL-36 expression to have higher absolute mean eosinophil counts compared with those with low IL-36 (2.6 vs 0.4 eosinophils per HPF, p = 0.21) (Table 2). While we also did not find statistically significant associations of IL-36 staining with histopathologic outcomes of eosinophil count and spongiosis, biopsies with high IL-36 staining had higher absolute mean eosinophil counts compared to those with low IL-36. Among other limitations of our study described in the primary analysis of this cohort,6 the large proportion of patients with high IL-36 staining would require a larger sample size to detect statistically significant differences. In addition, all patients with psoriasis in this study required treatment with biologic therapy, and thus, these histopathologic characteristics may not be generalizable to all patients with psoriasis. Of note, while approximately half (53%) of psoriasis biopsy specimens in our study did not have eosinophils, the remaining 47% contained eosinophils, with as many as over 20 eosinophils per HPF. These findings corroborate recent studies demonstrating that histopathologic findings of psoriasis can often include eosinophils,9 which could be driven in part by IL-36. Dermatopathologists should be aware of this traditionally underrecognized presence of eosinophils in psoriasis skin biopsies, which should not exclude a histopathologic diagnosis of psoriasis. The authors would like to thank Jamie Chan for her assistance and the UCSF Clinical and Translational Science Training Resident Research Funding Program for financial support. The authors declare no conflict of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.