Sequential Increase in Complement Factor I, iC3b, and Cells Expressing CD11b or CD14 in Cutaneous Vasculitis

Abstract

Mast cells contribute to the pathogenesis of cutaneous vasculitis through complement C3 that is cleaved to C3b and then to iC3b by complement factor I. The receptor of iC3b, CD11b, is expressed on neutrophils and monocytes and CD14 on monocytes. Their role in vasculitis is obscure. In this study, frozen skin biopsies from the nonlesional skin, initial petechial lesion, and palpable purpura lesion from 10 patients with immunocomplex-mediated small vessel vasculitis were studied immunohistochemically for complement factor I, iC3b, CD11b, and CD14. Peripheral blood mononuclear cells from 5 healthy subjects were used to study cell migration and cytokine secretion. Already, the nonlesional skin revealed marked immunostaining of complement factor I, iC3b, CD11b, and CD14, and their expression increased sequentially in initial petechial and palpable purpura lesions. Mast cell C3c correlated to iC3b, and both of them correlated to CD11b⁺ and CD14⁺ cells, in the nonlesional skin. The stimulation of mononuclear cells with 0.01-0.1 μg/ml iC3b induced cell migration in the transwell assay. C3a stimulated slightly interleukin-8 secretion, whereas 1 μg/ml iC3b inhibited it slightly, in 4/5 subjects. In conclusion, the C3-C3b-iC3b axis is activated already in the early vasculitis lesion leading to progressive accumulation of CD11b⁺ and CD14⁺ cells.

Figure 1

The immunohistochemical staining of iC3b (a–c) or CD11b (d–f) in the nonlesional skin (a, d), initial petechial lesion (b, e), and palpable purpura lesion (c, f) of a patient with vasculitis. The micrographs were taken using a 20x objective (scale bar 100 μm).

Figure 2

The correlation of (a) the number of C3c-positive mast cells (r = 0.726, p = 0.018) or (b) the staining score of iC3b (r = 0.798, p = 0.006) to the number of CD11b-positive cells in the nonlesional skin of 10 patients with vasculitis.

Figure 3

A sequential immunohistochemical staining of first complement factor I (a, c) and then mast cell tryptase (b, d). The skin biopsies were from the nonlesional skin (a, b) and palpable purpura lesion (c, d) of a patient with vasculitis. The micrographs were taken using a 20x objective (scale bar 100 μm).

Figure 4

A representative micrograph showing migrated peripheral blood mononuclear cells (PBMCs) on the lower surface of porous membrane after stimulation for 48 hours with (a) diluent control or (b) 25 μg/ml iC3b. The PBMCs, stained with May-Grünwald-Giemsa, were from the subject F29. The cells in Figure 4(b) belong mostly to the monocytic lineage, though they cannot be distinguished with absolute certainty from lymphocytes.

Figure 5

Stimulation of peripheral blood mononuclear cells from 5 healthy subjects for 24 hours using (a) iC3b and (b) C3a, and the resulting level of IL-8 in the conditioned medium in relation to the IL-8 level induced by the diluent control.