Plasmacytoid Dendritic Cells and Type I Interferon Signature in Lichen Striatus

Diagnostic utility of plasmacytoid dendritic cells in dermatopathology

Differentiating cutaneous diseases that mimic each other clinically and histopathologically can at times be a challenging task for the dermatopathologist. At the same time, differentiation of entities with overlapping features may be crucial for patient management. Although not seen in normal skin, plasmacytoid dendritic cells usually infiltrate the skin in several infectious, inflammatory/autoimmune and neoplastic entities. Plasmacytoid dendritic cells can be identified in tissue using specific markers such as CD123 and/or blood-derived dendritic cell antigen-2. Plasmacytoid dendritic cells are the most potent producers of type I interferons and their activity may therefore be assessed indirectly in tissue using human myxovirus resistance protein A, a surrogate marker for type I interferon production. In recent years, accumulating evidence has established the utility of evaluating for specific plasmacytoid dendritic cell-related parameters (plasmacytoid dendritic cell content, distribution and clustering and/ or human myxovirus resistance protein A expression) as a diagnostic tool in differentiating cutaneous diseases with overlapping features such as the alopecias, lupus and its mimics, and neoplastic entities. In this review, we provide an update on the current evidence on this topic and on the contexts where this can be a useful adjunct to reach the histopathological diagnosis.

Plasmacytoid dendritic cells in dermatology

Plasmacytoid dendritic cells are part of the dendritic cells family and are a relevant link between innate and adaptive immunity. They are the most potent producers of type 1 interferon, generating antiviral response, stimulating macrophages and dendritic cells and inducing activation and migration of natural killer cells. Plasmacytoid dendritic cells also exert a role as antigen-presenting cells, promote T-lymphocyte responses, immunoregulation, plasma cells differentiation and antibody secretion. Even though plasmacytoid dendritic cells are not usually present in normal skin, their presence is detected in healing processes, viral infections, and inflammatory, autoimmune, and neoplastic diseases. In recent years, the presence of plasmacytoid dendritic cells in several dermatological diseases has been described, enhancing their potential role in the pathogenesis of such conditions. Future studies on the role of plasmacytoid dendritic cells in dermatology may lead to new therapeutic targets.

Plasmacytoid dendritic cells in keratoacanthoma and squamous cell carcinoma: A blinded study of CD123 as a diagnostic marker

BACKGROUND

Histopathologic distinction between keratoacanthoma (KA) and squamous cell carcinoma (SCC) is challenging. We surmised that a discriminatory immunostain would be clinically meaningful. Previous investigators have found CD123-positive plasmacytoid dendritic cells (PDCs) are more prominent in KA than SCC. We sought to determine if CD123 immunostaining might have value as a diagnostic test for distinguishing KA from SCC.

METHODS

We used blinded, semi-automated image analysis to compare CD123 expression in 66 KAs and 63 SCCs in a tissue microarray.

RESULTS

PDCs were present in both KA and SCC. Mean PDC frequency was higher in KA than SCC (14.2 vs 11.2 mean cells/0.0945 square mm) but the difference was not statistically significant (P = 0.1240). There was no significant difference in mean PDC cluster frequency, mean intratumoral PDC frequency, or the percentage of PDCs as proportion of the total mononuclear inflammatory cell infiltrate between KA and SCC.

CONCLUSION

CD123 immunostaining is not a clinically useful test for distinguishing KA from SCC.

Possible role of plasmacytoid dendritic cells in pityriasis lichenoides

BACKGROUND

Plasmacytoid dendritic cells (pDCs) and their product, type I interferons (IFNs), have been implicated in the pathogenesis of several skin disorders characterized by an interface dermatitis (ID) pattern, such as lichen planus (LP). A type I IFN signature has previously been documented in pityriasis lichenoides (PL). Although pDCs are known to be the main source and most potent producers of local type I IFNs, their role in PL has not been investigated.

AIM

To investigate the role of pDCs in PL.

METHODS

In total, 20 cases of PL and 20 comparable cases of LP were immunohistochemically tested for pDC occurrence and type I IFN production using anti-blood-derived dendritic cell antigen-2 (BDCA2; a specific pDC marker) and anti-myxovirus protein A (anti-MxA) antibodies (indirect marker of pDC activity), respectively. MxA is a well-established surrogate marker for local type 1 IFN production. A semiquantitative scoring system was used.

RESULTS

pDCs were present in all 40 cases with no statistically significant difference between the two groups. MxA expression was intense and diffuse in the majority of PL and LP cases.

CONCLUSIONS

pDCs constitute a central component of the inflammatory infiltrate in PL, suggesting that PL shares with the other entities that exhibit an ID a common pDC-driven process through type I IFN production, which ultimately leads to the cytotoxic attack.