Halo Nevus – the Vascular Connection

Material and Methods: A random series of 137 nevi, including 75 HN were examined. Serial paraffin and frozen sections were subjected to: routine histochemistry; (HE, Reticulin, Auro, PAS), enzyme histochemistry: dopa oxidase, counterstained with Nuclear Fast Red (NFR) to highlight the endothelium and dopamine oxidase; immunohistochemistry to assess presence of lymphocytic infiltrates; electron microscopy: after enbloc dopa stain.

The causes of both vitiligo and halo nevi are poorly understood. An immune-mediated process has been suggested. Four sequential stages of mononuclear infiltrate of macrophages, cytotoxic T cells, and Langerhans cells have been described [13,19,20].
Patients with halo nevi have circulating antibodies to cytoplasmic antigens in melanoma cells [19,21]. Halo reaction can also cause increased cytologic atypia in lesional melanocytes and obscure the architectural features of the underlying nevus, making distinction between a benign melanocytic nevus and melanoma difficult [10]. The mode of involution of nevi has been studied in depth since the present series of cases shows a complete absence of infiltrates.

Material and Methods
The present study includes a random series of 137 Nevi, 75 being Halo Nevi associated vitiligo, received from the Dermatology Unit of Safdarjung Hospital, New Delhi, fixed in 10% formol glutaraldehyde.

Halo nevi
All 75 halo nevi were assessed for the process of loss of pigmentation. The lesions show a range of dissolution from initial stages to almost complete depletion of the component cells in the nevus. The periphery shows loss of epidermal melanocytes forming the halo. Occasional pigmented nevi regress completely and are replaced by a vitiliginous patch. One case shows a halo around a capillary hemangioma. (Figures  2-4).

Individual cases
The cases show varying extents of involution and are graded into three groups, 1. Early stage; 2. Moderate dissolution; 3. Complete dissolution. Three random cases in each group are described below.

Progress of involution
All 75 halo nevi were assessed for the process of loss of pigment cells. The lesions show a range of dissolution from initial stages to almost complete depletion of the component cells in the nevus. The periphery shows loss of epidermal melanocytes forming the halo. Occasional pigmented nevi regress completely and are replaced by a depigmented vitiliginous patch. The nevus cells do not show necrosis, the NFR positive cells showing viable nuclei. This is corroborated by the absence of reactive infiltrates. Contrary to expectation, on careful scrutiny on HE as well as immunohistochemistry there are no inflammatory infiltrates in any of the samples studied ( Figure 5).
Most of the HN are IDN or CpN with initial abundance of nevus cells within the dermis, separated by a Grenz zone in IDN or reaching up to the epidermis and mingling with epidermal melanocytes. Initial changes are in the form of spaces formed within the peripheral and in-depth groups of nevus cells. (Figure 6Ia). On dopa-NFR staining a gradual loss of pigment and dopa is seen with increase in NFR in the lining cells. The process continues to replace cells lining the spaces with NFR positive endothelial cells. As the process extends the nevus cells    are gradually replaced by conjeries of endothelial lined spaces which connect with underlying vasculature. The vessels finally merge and are remodeled to blend with the dermal vessels (Figure 6Ib). In some compact nevus cells are replaced by spaces resembling vasculogenic sinusoids (Figure 6Ic). (Figure 6Id) shows complete dissolution with remnants of dopa positive cells lining thin walled vessels.
Initially spaces lined by flatten dopa positive nevus cells appear at the periphery and depth of the lesion. These do not show PAS or reticulin positive basement membranes. On dopa-NFR staining, a graded loss of dopa is associated with increase in NFR in the flattened lining cells. The nevus cells are replaced by spaces lined with NFR positive flattened cells which are continuous with the endothelial lining of the underlying vasculature to finally merge, and blend with the dermal vessels. The section in (Figure 6II) shows the stepwise involution and replacement by vascular channels. On the right complete involution is seen, the vessels merging with the dermis and reformation of fibrous tissue. The left shows early stages of vascular change. The camera lucida tracings highlight the process ( Figure 6IICL).
A similar process is evident in marginal melanocytes. The epidermal melanocytes surrounding the nevus are gradually lost to form a halo. The nevus cells and the epidermal melanocytes are replaced by endothelial cells to result in the involution of the nevus. The process simulates vasculogenesis seen in amelanotic melanomas. Angiogenesis is not elicited from the stromal pre-existing vasculature as observed in melanomas. Thus the nevus cells and the epidermal melanocytes are replaced/converted into endothelial cells to result in the involution of the nevus. The appearance of a halo around a capillary hemangioma is of interest as it links the replacement of nevus cells by endothelial cells with resulting involution.

Discussion
HN usually appears on the back and is common in children and young adults, with a mean age of onset at 15 years [6,27,28]. The incidence of HN in the population is estimated to be approximately 1% and there is no predilection for sex or race [3,29]. Multiple lesions are found in about 50% of the cases.
Four clinical stages have been described beginning with a surrounding rim of depigmentation around a pigmented nevus (stage I), the central nevus loses its pigmentation (stage II) and disappears leaving a circle of depigmentation (stage III). In stage IV the depigmented area undergoes repigmentation, over a period of months or even years [6,9,30]. Most lesions are compound, junctional, or intradermal nevi [8]. Halo nevi, are often associated with concurrent vitiligo [8].
The causes of both vitiligo and halo nevi are poorly understood. An immune-mediated process has been suggested resulting from damage or destruction of melanocytes. Four sequential stages of mononuclear infiltrate of macrophages, cytotoxic T cells, and Langerhans cells have been described [7,13,19,20,30,31].
Patients with halo nevi may have circulating antibodies to cytoplasmic antigens in melanoma cells which disappear upon excision or spontaneous resolution of the central lesion [19,21,32]. Halo reaction can also cause increased cytologic atypia in lesional melanocytes and obscure the architectural features of the underlying nevus, making distinction between a benign melanocytic nevus and melanoma difficult [10].
In the present study, it is observed that occasional pigmented nevi regress completely and are replaced by a depigmented vitiliginous patch. Contrary to expectation, the present series of 75 HN (51 + 24 cases) shows no evidence of inflammatory infiltrates in any of the samples studied at any stage. There are nevi showing a range from initial stages to almost complete depletion of pigment cells in the nevus. The associated epidermis shows loss of peripheral epidermal melanocytes forming the halo.
Most of the HN are Intradermal (IDN) or Compound Nevi (CpN) with initial abundance of nevus cells within the dermis, separated by a Grenz zone in IDN or reaching up to the epidermis and mingling with epidermal melanocytes in CpN. Initial changes are in the form of spaces formed within the peripheral and in-depth nevus cells. The nevus cells flatten to line the spaces. On dopa-NFR staining a gradual loss of pigment and dopa is seen with increase in NFR in the lining cells first in the nuclei followed by the cytoplasm. The process is very similar to vasculogenesis seen in amelanotic melanomas.
The process continues to replace cells lining the spaces with NFR positive endothelial cells. There is no definite evidence of nevus cell necrosis. As the process extends the nevus cells are gradually replaced by conjeries of endothelial lined spaces which connect with underlying vasculature. The vessels finally merge and blend with the stromal vessels. There is no evidence of angiogenesis from the pre-existing stromal vessels or development of Tumor Vascular Complexes (TVCs) as observed in melanomas as seen during tumor angiogenesis [33].
In HN the replacement of melanocytes by endothelium results in involution of the lesion and remodeling of the vascular channels which merge with the pre-existing connective tissue stroma. It is difficult to explain the extremely rare cases of concurrent regression of melanoma on this basis. Two factors are likely to be involved: firstly, a rapid replacement of tumor cells by extensive vascularisation. Secondly, patients with halo nevi may have circulating antibodies to cytoplasmic antigens in melanoma cells which disappear upon excision or spontaneous resolution of the central lesion [19,21,32].