Primary Localized Cutaneous Amyloidosis: A Histological and Immunohistochemical Appraisal

Abstract

Background:

Primary localized cutaneous amyloidosis (PLCA) is a rare chronic heterogeneous group of disorders with deposition of abnormally folded beta-pleated protein in the dermo-epidermal junction and dermis. There are three major forms of PLCA, including lichen amyloidosis, macular amyloidosis, nodular amyloidosis, and a rare variant, amyloidosis cutis dyschromica. We aimed to evaluate the role of immunohistochemistry (IHC) in the diagnosis of PLCA.

Patients and Methods:

A retrospective 8-year audit (2017–2024) was performed. All cases of PLCA were subjected to a panel of IHC, including high molecular weight cytokeratin (HMWCK), Kappa and Lambda light chains; and serum amyloid A. The demographic details and clinical features, along with site predilection, were correlated with the pathological subtypes.

Results:

We included 42 cases of PLCA, among which lichen amyloidosis was the most common subtype (50%), followed by macular amyloidosis, nodular amyloidosis, and amyloidosis cutis dyschromica, with a predilection for the upper back. IHC displayed HMWCK positivity in cases of lichen amyloidosis (14/21, 66.7%), macular amyloidosis (7/11, 63.6%), and amyloidosis cutis dyschromica (2/3, 66.7%). All cases of nodular amyloidosis showed light chain restriction using a combination of IHC and direct immunofluorescence.

Limitations:

Retrospective nature of the study, relatively small sample size, and not performing IHC for other small molecular weight keratins.

Conclusion:

The findings in our study confirm that the amyloid in macular amyloidosis, lichen amyloidosis, and amyloidosis cutis dyschromica is derived from epidermal keratinocytes. IHC for HMWCK should be part of diagnostic workup as it can detect very small amount of amyloid deposition, which may be overlooked during routine evaluation. Nodular amyloidosis demonstrates light chain restriction, indicating the need to exclude systemic amyloidosis.

Introduction

Amyloid is an extracellular proteinaceous deposit arising from protein misfolding resulting in resistance to proteolytic digestion. It is characterized by pale, eosinophilic, amorphous, glassy, periodic acid-Schiff (PAS) negative deposits on histopathological sections and displays congophilia with birefringence properties under polarized light.[1] Amyloid deposits can either be systemic or localized. The cutaneous deposits of amyloid could be due to systemic disease that could be primary (secondary to plasma cell dyscrasia) or secondary [like serum amyloid A (SAA) amyloidosis]. However, when the deposition is localized to the skin without systemic deposition or primary clonal disorders, it is labelled as primary localized cutaneous amyloidosis (PLCA).[2] The four defined histological subtypes of PLCA include lichen amyloidosis, macular amyloidosis, nodular amyloidosis, and amyloidosis dyschromica.[3] The nature of cutaneous amyloid deposition is not well characterized, and a handful of previous studies have tried to explore the nature of it. Most of the previous studies have shown that the amyloid in PLCA is derived from keratin. The present study aims to evaluate the role of immunohistochemistry (IHC) for the diagnosis and characterization of PLCA.

Patients and Methods

All cases of cutaneous amyloidosis diagnosed on skin biopsy over an 8-year period (2017–2024) were retrieved from the electronic database of the Department of Histopathology, Postgraduate Institute of Medical Education and Research, Chandigarh, India. This study was approved by the institute ethics committee (reference no. IEC-INT/2025/Study-3311). The clinical details of these cases were obtained. The cases showing systemic amyloidosis with secondary cutaneous involvement were excluded from the analysis. Only cases established as PLCA based on clinicopathological correlation were included in this study.

For all cases of PLCA, the slides (Hematoxylin and Eosin [H&E] and Congo Red) were retrieved and analyzed by two pathologists for the confirmation of the diagnosis. For all cases where blocks were available, a panel of IHC markers was performed [Supplementary Table 1].

Supplementary Table 1.

Panel of immunohistochemistry

Antibody name	Company	Clone	Dilution
Kappa	Cell Marque	L1C1	1:300
Lambda	Cell Marque	L1C1	1:300
High molecular weight cytokeratin (HMWCK)	DAKO	34 beta E12 (CK1, CK5, CK10, CK14, CK903)	1:100
SAA (serum amyloid A)	DAKO	MC1	1:15

All cases of nodular amyloidosis were also subjected to direct immunofluorescence (DIF) for kappa and lambda light chains. DIF was performed on formalin-fixed, paraffin-embedded (FFPE) tissue after proteinase K digestion.

No formal sample size calculation was carried out as the exact prevalence of cutaneous amyloidosis is not known and all cases of cutaneous amyloidosis are not biopsied.

Statistical analysis

Statistical analysis was primarily descriptive. Continuous variables were summarized as mean and range, while categorical variables were presented as frequencies and percentages. Clinical, histopathological, and immunohistochemical findings were analyzed using counts and proportions. Diagnostic concordance between clinical and histopathological diagnoses was calculated as percentage agreement. Data analysis and tabulation were performed using Microsoft Excel.

Results

Over this period, 42 cases of PLCA were diagnosed, with an average age of onset at 43 years (range: 19–68 years). There were equal number of male and female patients (21 each, M:F ratio 1:1). The most common site of localization was upper back (n = 20, 47.6%), followed by shins (n = 11, 26.1%), and arms (n = 6, 14.2%) [Supplementary Table 2]. The predominant clinical presentation included hyperpigmented papules measuring 2–4 mm. History of itching was present in 23 (54.8%) patients. Among the different subtypes, lichen amyloidosis was the most common subtype (n = 21, 50%), followed by macular amyloidosis (n = 11, 26.2%), nodular amyloidosis (n = 7, 16.6%), and amyloidosis dyschromica (n = 3, 7.1%) [Figure 1]. The clinical features are highlighted in Supplementary Table 2.

Supplementary Table 2.

Clinical features of the patients (n=42)

Mean age		43 years (range: 19–68 years)
Sex	Male	21
	Female	21
	Male: Female	1:1
Site	Upper back	20 (47.6%)
	Shins	11 (26.1%)
	Arms	6 (14.2%)
	Infraorbital region	1 (2.6%)
	Buttock	1 (2.6%)
	Nose	1 (2.6%)
	Clavicle+scapular region	1 (2.6%)
	Chest wall	1 (2.6%)
Type of PLCA	Lichen amyloidosis	21 (50%)
	Macular amyloidosis	11 (26.2%)
	Nodular amyloidosis	7 (16.6%)
	Amyloidosis cutis dyschromica	3 (7.1%)

PLCA=Primary localized cutaneous amyloidosis

Figure 1.

Clinical pictures of cutaneous amyloidosis. (a) Macular brownish pigmentation with rippling on left shin. Skin is atrophic possibly because of long-term topical corticosteroid use. (b) Dirty looking brownish closely set hyperkeratotic papules. Rippling can be appreciated at places. (c) Hyperkeratotic brownish papules on shin with lichenification and post-inflammatory depigmentation due to chronic scratching

A concordance rate of 83.3 % (n = 35) was noted with the clinical and histopathological examination diagnosis.

Histologically, all the PLCA were characterized by deposition of pale, eosinophilic, amorphous, and glassy material limited to the papillary dermis [Figure 2]. The amount of amyloid deposition was variable. In 14 (33.3%) cases, it was very focal and subtle. Some cases had more prominent amyloid deposition, which was at places extending into upper part of reticular dermis. In addition, the lichen amyloidosis cases (n = 21) commonly exhibited acanthosis (n = 19, 90.5%), perivascular lymphocytic infiltrate (n = 12, 57.1%), and pigment incontinence with presence of melanophages (n = 14, 66.7%). Hyperkeratosis and parakeratosis (n = 12, 57.1%) were more prevalent in lichen amyloidosis compared to other subtypes. In these cases, having hyper and parakeratosis, the granular cell layer was normal or showed hypogranulosis, consistent with chronic scratching and friction-induced epidermal damage. Macular amyloidosis showed a relatively scanty amount of amyloid in the papillary dermis and displayed mild acanthosis (n = 4, 36.4%), hyperkeratosis and parakeratosis (n = 4, 36.4%) in a subset of cases. In nodular amyloidosis, there was nodular deposition of amyloid in the dermis [Figure 3]. In this group, a consistent additional histopathological finding was perivascular lymphoplasmacytic infiltrate (n = 4, 57.1%), and acanthosis (n = 3, 42.9%). The histopathological findings are summarized in Table 1.

Figure 2.

(a) Histology in a case of macular amyloidosis showing deposition of small amount of amyloid in the papillary dermis (arrow) (Hematoxylin and eosin, 100x). (b) The corresponding deposit (arrow) is being highlighted on high molecular weight cytokeratin staining (immunoperoxidase, x100). (c) A case of macular amyloidosis showing deposition of diffuse pale eosinophilic amyloid in the upper dermis with pigment incontinence (Hematoxylin and eosin, 100x). (d) Higher magnification showing amyloid in the upper dermis (Hematoxylin and eosin, 200x). (e) Congo red staining highlight congophilic amyloid deposition (arrow) in the upper dermis (x100). (f) High molecular weight cytokeratin immunostaining in the same case shows diffuse amyloid deposition both in upper as well as mid dermis (immunoperoxidase, 100x)

Figure 3.

(a) Histology of a case of nodular amyloidosis shows deposition of abundant pale, eosinophilic amyloid throughout the dermis (Hematoxylin and eosin, 100x). (b) Apple green birefringence is seen on polarized microscopy on Congo red staining (100x). On performing direct immunofluorescence, this case showed kappa restrictions [(c) kappa light chain, (d) lambda light chain, 100x]

Table 1.

Histopathological characteristics of primary localized cutaneous amyloidosis

Histopathological characteristics	Diagnosis
	LA (n=21)	MA (n=11)	NA (n=7)	AD (n=3)
	N (%	N (%)	N (%)	N (%)
Acanthosis	19 (90.5%)	4 (36.4%)	3 (42.9%)	1 (33%)
Hyper granulosis	0	1 (9.1%)	0 (0%)	0 (0%)
Hyperkeratosis and parakeratosis	12 (57.1%)	4 (36.4%)	2 (28%)	1 (33%)
Perivascular lymphoplasmacytic infiltrate	12 (57.1%)	3 (27.3%)	4 (57.1%)	1 (33%)
Pigment incontinence	14 (66.7%)	4 (36.4%)	1 (14%)	1 (33%)
HMWCK	14 (66.7%)	7 (63.6%)	0	2 (66.7%)
SAA	0	0	0	0
Kappa light chain	0	0	4 (57.1%)	0
Lambda light chain	0	0	3 (42.9%)	0

AD=Amyloidosis dyschromica; HMWCK=High molecular weight cytokeratin; LA=Lichen amyloidosis; MA=Macular amyloidosis; NA=Nodular amyloidosis; SAA=Serum amyloid A

IHC displayed HMWCK positivity in cases of lichen amyloidosis (14, 66.7%), macular amyloidosis (7, 63.6%), and amyloidosis dyschromica (2, 66.7%). HMWCK positivity was more extensive compared to Congo red staining, and in many cases, it could be seen extending into reticular dermis. In three cases, HMWCK highlighted very scanty amount of amyloid in the papillary dermis, which was not appreciable on light microscopy or Congo red staining. None of the variants showed positivity for SAA. Among the nodular amyloidosis cases (n = 7), three cases each (42.8%) showed kappa and lambda light chain restriction on IHC, which was confirmed on DIF [Table 1]. In one nodular amyloidosis case, IHC for both kappa and lambda light chains was negative. However, this case revealed kappa restriction on DIF. None of the nodular amyloidosis cases showed positivity for HMWCK.

Out of 7 cases of nodular amyloidosis, follow-up information was available for 6 cases (range: 13 months to 7 years). There was no evidence of light chain restriction on serum protein electrophoresis or free light chain assay in these patients. None of these patients developed systemic amyloidosis or plasma cell dyscrasia over the follow-up period.

Discussion

PLCA represents localized cutaneous deposition of amyloid fibrils in the absence of clonal hematopoiesis. This entity comprises four histological subtypes- lichen amyloidosis, macular amyloidosis, nodular amyloidosis, and amyloidosis dyschromica.[3] Amyloidosis cutis dychromica is a rare subtype for which only around 50 cases have been reported worldwide till date.[4,5] The diagnosis of the amyloidosis subtype and, consequently, its treatment are based on distinctive clinical, histopathologic, and laboratory findings. In line with existing literature, lichen amyloidosis was the most prevalent subtype in our study, followed by macular amyloidosis, nodular amyloidosis, and amyloidosis dyschromica. Vijay et al.[6] reported 65.63% of lichen amyloidosis and 15.63% of macular amyloidosis in their series.

There is no substantial evidence regarding the origin of amyloid in the skin. Two theories have been proposed, fibrillar body theory and secretory theory. The fibrillar body theory suggests degenerated epidermal keratinocytes as the source of PLCA. This is supported by ultrastructural studies showing tonofilament deposition morphologically resembling amyloid filaments and exhibiting IHC positivity for keratin similar to epidermal keratinocytes.[7] The secretory theory states that the amyloid deposition is a result of the dropping of basal epidermal keratinocytes into the papillary dermis.[8]

IHC subtyping and categorization of primary cutaneous amyloidosis have been attempted in some previous studies. Huilgol et al. noted that all the seven cases of lichen and macular amyloidosis tested positive for LP34 (which labels CK5, CK6, and CK18), MNF 116 (which labels CK5, CK6, CK8, CK17, and CK19), and RCK 102 (which labels CK5 and CK8).[9] Similarly, Inoue et al. also observed 33 of 39 (84.6%) cases showed positivity for 34 beta E12 (CK1, CK5, CK10, and CK14) and 32 (82.1%) cases for MNFl16 in lichen and macular amyloidosis cases.[10] Sari Aslani et al. evaluated 42 cases of macular amyloid for HMWCK IHC, and found it to be positive in around 50% cases, while Congo red staining was inconspicuous.[11] These findings strongly support the idea that skin-limited amyloid originates from basal keratinocytes. As CK5 was a common antigen detected by these antibodies, and it is predominantly present in basal keratinocytes, it was proposed as an ideal candidate for IHC in PLCA.[9,10] Similar findings have been reported by few previous Indian studies.[12,13,14] The presence of CK7, CK17, and CK19 within the amyloid, which are absent in the interfollicular epidermis, indicates an appendageal origin of the amyloid deposits.[15] Sidey et al.[16] reported a case of cutaneous squamous cell carcinoma (SCC) with concomitant amyloid deposition being treated with Moh’s micrographic surgery. On frozen section, amyloid also stained with HMWCK, which could potentially mimic residual SCC. Hence, HMWCK IHC should be interpreted with caution, especially on frozen section in the setting of malignancy.

However, it is important to note that in our study, not all cases tested positive for HMWCK, suggesting that some amyloid deposits may contain a different set of keratins other than CK1, CK5, CK10, and CK14.

However, another possibility could be a very tiny amount of amyloid deposit in the biopsy in some of the cases of macular and lichen amyloidosis, making IHC interpretation difficult. We also came across three cases, where amyloid could not be detected initially on Congo red staining; however, HMWCK IHC detected tiny amount of amyloid in the papillary dermis. Crystal violet is more sensitive compared to Congo red for detection of cutaneous amyloid, especially in the sun damaged skin. Thus, IHC may be helpful in identifying tiny amounts of amyloid deposition. While IHC was negative in one-third of cases, it detected amyloid in three patients where Congo red was non-contributory. IHC for HMWCK also demonstrated more extensive amyloid deposition than could be detected by hematoxylin and eosin staining or Congo red staining. Thus, IHC provides incremental diagnostic yield, especially for tiny/focal deposits that may be missed on routine stains. We justify its inclusion as an adjunct rather than a replacement for histology and Congo red.

In our study, nodular amyloidosis cases showed almost an equal distribution between kappa and lambda light chain restriction. One case that tested negative in IHC exhibited kappa light chain restriction in DIF. Clonality and the presence of plasma cells suggest a potential low-grade B-cell lymphoproliferative disease in these cases, warranting more aggressive follow-up.[17] However, cases in our series did not show any evidence of systemic disease on reasonably long follow-up.

Limitations

The limitations of the study are its retrospective nature, relatively small sample size, as biopsy is carried out primarily in cases with clinical differential diagnoses and not in classical cases. Also, we did not perform IHC for other small molecular weight keratins for the characterization of cutaneous amyloidosis. Another limitation of our study is that DIF was performed on FFPE sections due to the retrospective nature of the study. Although frozen sections are preferable because they preserve antigenicity better, DIF on FFPE yielded reliable results and showed concordance with IHC in most cases.

Conclusion

The findings in our study confirm that the amyloid in macular amyloidosis, lichen amyloidosis, and amyloidosis cutis dyschromica is derived from epidermal keratin. IHC for HMWCK should be part of diagnostic workup as it can detect very small amount of amyloid deposition, which may be overlooked during routine evaluation. Nodular amyloidosis demonstrates light chain restriction, indicating the need to exclude systemic amyloidosis.

Author contributions

We confirm that all authors have read and approved the manuscript for submission. All authors meet authorship criteria, and the manuscript represents honest work.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Conflicts of interest

One of the authors is editor of the journal. The manuscript was subject to journal’s standard procedure and peer review was held independently of the editor or their research group.

Use of artificial intelligence (AI)

The preparation of this manuscript was carried out entirely by the authors without the use of artificial intelligence technologies.

Funding Statement

Nil.