The most common subtype of cutaneous T-cell lymphoma, mycosis fungoides (MF), is characterized by proliferation of malignant T cells in the skin. In the more clinically aggressive folliculotropic MF (FMF), malignant T cells localize to follicular epithelium, whereas in classic MF, they infiltrate the dermis and epidermis. How the localization of neoplastic T cells to the follicular niche contributes to the clinical aggression in FMF is unclear. To uncover the tumor microenvironmental differences between perifollicular FMF and dermal classic MF regions, we analyzed patient samples using spatial transcriptomics. Transcripts were collected from specific cell subsets within follicular (FMF) and dermal (classic MF) regions of interest to compare gene expression, spatial deconvolution, and pathway activity. Our work revealed that the neoplastic CD4 T cells around the hair follicles in FMF had a highly inflammatory phenotype, better adaptation to cellular starvation, higher metabolic activity, and enhanced antigen presentation. In contrast, cutaneous T-cell lymphoma-associated macrophages in FMF exhibited an immunosuppressive phenotype with decreased IL-2 and IFN signaling. These findings suggest that the follicular microenvironment may provide survival advantages to malignant T cells while promoting a dysregulated antitumor immune response. These observations provide insight into the mechanisms underlying the more aggressive clinical features of FMF versus classic MF.

A 38-year-old male presented with waxy papules, plaques over the neck and extremities, and ichthyotic scales over the lower limbs. Skin biopsy revealed a dense medium-sized lymphocytic infiltrate in the dermis, with perifollicular accentuation and focal exocytosis into the follicular epithelium with strong positivity for CD 3, 4, and 5. Considering the clinicopathological correlation, a diagnosis of follicular mycosis fungoides (FMF) was made. It is a variant of classic mycosis fungoides (MF) where atypical cells invade the follicular epithelium.

Programmed cell death protein 1 (PD-1) plays a pivotal role in immune system regulation, with its expression levels linked to malignancy prognosis. However, existing reports on PD-1 staining in mycosis fungoides (MF) present conflicting findings, and little attention has been given to PD-1 staining in different MF variants. To address this, we conducted a retrospective study, employing immunohistochemistry to examine PD-1 expression in cases of folliculotropic MF and non-folliculotropic MF. We analyzed 24 cases of folliculotropic MF and 18 cases of non-folliculotropic MF, and recorded both the percentage of PD-1-labeled tumor cells and the intensity score (negative, weak, medium, or strong). Our results revealed significant disparity in PD-1 labeling between patch/plaque MF and folliculotropic MF (p = 0.028). Non-folliculotropic MF exhibited higher PD-1 labeling in tumor cells (58.3%) compared to folliculotropic MF (40.2%). Notably, there was no significant difference in PD-1 staining between folliculotropic MF and non-folliculotropic MF when both were in the early stage/indolent disease category. However, when considering the tumor stage, folliculotropic MF exhibited PD-1 staining in tumor cells at a rate of 21.1%, while non-folliculotropic MF showed PD-1 staining in tumor cells at a rate of 46.6% (p = 0.005). Additionally, among folliculotropic MF cases, 13 out of 24 cases displayed differing PD-1 expression patterns between epidermal and dermal components, with preserved PD-1 staining in the epidermal component and loss of staining in the dermal component. Furthermore, consistent with the prior literature, tumor cells with large cell transformations exhibited significantly lower PD-1 labeling (p = 0.017). Our findings showcase the unique PD-1 staining patterns in MF.

Two patients presented with erythematous papules within larger patches and thin plaques. Following biopsies, each case was initially thought to represent interstitial granulomatous dermatitis (IGD); however, clinicopathological correlation led to a diagnosis of granulomatous mycosis fungoides (GMF). Drawing upon the similarities between these cases, this report explores the clinical and histological manifestations of GMF, features distinguishing GMF from other granulomatous diseases like IGD and the prognostic significance of distinguishing GMF from classic mycosis fungoides. This report also shows that despite the potential for histological overlap between GMF and IGD, the existing literature does not reveal an epidemiological or pathophysiological link between these two conditions.

Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) is associated with an increased risk of a second malignancy. We conducted a retrospective clinicopathologic review of 12 patients with CLL/SLL who developed a second lymphoma in the skin. Demographic data, clinical information, and histopathology from 31 biopsies were recorded. Cases of secondary cutaneous involvement by CLL/SLL (leukemia cutis) and non-primary cutaneous lymphomas were excluded. A wide variety of primary cutaneous lymphomas was identified, including classic mycosis fungoides (3), cutaneous marginal zone lymphoma (2), primary cutaneous peripheral T-cell lymphoma unspecified (2), folliculotropic mycosis fungoides (1), Sézary syndrome (1), cutaneous gamma-delta T-cell lymphoma (1), cutaneous CD8+ aggressive epidermotropic cytotoxic T-cell lymphoma (1), and cutaneous anaplastic large cell lymphoma (1). A male predominance was observed, and the average age was 74.1 years. In all patients, CLL/SLL predated the development of the second lymphoma, which was aggressive in the majority of cases (58%). Aggressive cytotoxic T-cell lymphomas, generally rare neoplasms, were relatively common (30%). CLL/SLL patients may develop a second lymphoma in the skin, which may be aggressive. Atypical cutaneous lymphoid infiltrates in this patient population should not be assumed to represent secondary CLL/SLL involvement and require thorough immunohistochemical analysis.