Mixed Connective Tissue Disease

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Abstract
A study of the cutaneous eruptions of eight patients with mixed connective tissue disease (MCTD) was performed to better characterize its dermatopathology and to explore a role for the membrane attack complex of complement C5b-9 in lesional pathogenesis. Nine lesional skin biopsies were obtained from eight patients with MCTD and analyzed by conventional light microscopy. Direct immunofluorescence (IF) and indirect IF using a monoclonal antibody to C5b-9 were applied in six and five cases respectively. The biopsied cutaneous eruptions were characterized clinically as photodistributed erythematous annular and/or papulosquamous lesions mimicking subacute cutaneous lupus erythematosus (SCLE) in five of eight patients as an ill-defined, telangiectatic, scaly patch on the face in one patient, palpable purpura in one patient, and dorsal hand blisters resembling porphyria cutanea tarda (PCT) in another. With the exception of the latter two patients, the histology appeared similar, comprising a cell poor and/or lichenoid interface dermatitis with suprabasilar exocytosis around necrotic keratinocytes in the absence of deep periadnexal or perivascular extension or conspicuous follicular plugging, a pattern similar to that of SCLE. However, the lesions differed from SCLE by virtue of vasculopathic alterations comprising vascular ectasia, hypovascularity, and/or luminal thrombosis confined to the superficial vascular plexus and a sclerodermoid tissue reaction, the latter seen in two cases. One biopsy showed a pustular leukocytoclastic vasculitis (LCV). In another case, a biopsied hand blister demonstrated a PCT-like appearance histologically, namely, pauci-inflammatory subepithelial blister formation with hyalinization of dermal papillae capillaries accompanied by an LCV. There was nuclear keratinocyte decoration with IgG and C5b-9 in all cases studied, accompanied by a positive lupus band test in two cases and homogenous deposition of immunoreactants along the dermoepidermal junction and within vessels in the PCT-like eruption. Granular vascular decoration with immunoreactants including C5b-9 was seen in two LCV cases and in two biopsies from rashes clinically mimicking SCLE. Although the epidermal pathology of MCTD mimicks that of SCLE, a concomitant vasculopathy paralleling that seen in skin lesions of dermatomyositis distinquishes the dermatopathology of MCTD from that of SCLE. Corroborating the role of microangiopathy in the pathogenesis of the skin lesions of MCTD was the demonstration of C5b-9 in blood vessels. The deposition of C5b-9 in keratinocytes may explain the pattern of IgG decoration of keratinocytes; the formation of plasmalemmal pores may permit binding of immunoglobulin to antigens in the nucleus and/or cytosol. The C5b-9 complex may be the effector mechanism of epithelial and/or endothelial cell injury in MCTD or may serve to augment the effects of antibody-dependent cellular cytotoxicity.

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