IgG4 autoantibodies induce dermal-epidermal separation

Abstract

Bullous pemphigoid (BP) is a sub-epidermal autoimmune blistering disease associated with autoantibodies to the dermal-epidermal junction (DEJ). Patients' autoantibodies induce dermal-epidermal separation when co-incubated with cryosections of human skin and leucocytes from healthy volunteers. IgG autoantibodies trigger complement and/or leucocyte activation resulting in specific pathology in several autoimmune conditions. In these diseases, IgG1 and IgG3 isotypes, but not the IgG4 subclass, are thought to trigger inflammatory pathways resulting in tissue damage. The capacity of IgG4 autoantibodies to mediate tissue damage has not yet been demonstrated. In this study, we isolated IgG1 and IgG4 autoantibodies from bullous pemhigoid patients'serum and analysed their blister-inducing potential in our cryosection assay. As expected, complement-fixing IgG1 autoantibodies induced sub-epidermal splits in this experimental model. Purified IgG4 did not fix complement, but, interestingly, like IgG1, activated leucocytes and induced dermal-epidermal separation. The potential of IgG4 autoantibodies to induce Fc-dependent dermal-epidermal separation was significantly lower compared to IgG1. Our results demonstrate that IgG4 autoantibodies are able to activate leucocytes and point to a hitherto less recognized function of IgG4. Moreover, for the first time, we clearly demonstrate that BP IgG4 autoantibodies have the capacity to induce leucocyte-dependent tissue damage.

1

Purification of IgG1 and IgG4 autoantibodies specific to the epidermal basement membrane. IF microscopy study of serum from patient 1 on normal human skin demonstrates deposition of mainly IgG1 (A) and IgG4 (D) and traces of IgG2 (B) and IgG3 (C) at the dermal–epidermal junction. After purification against an affinity matrix containing mon-oclonal antibodies specific for IgG1, IgG1 autoantibodies to the skin basement membrane can be visualized in the purified fraction (E), while IgG2 (F), IgG3 (G) and IgG4 (H) autoantibodies are not detectable. Conversely, the antibody fraction purified against an affinity matrix specific for IgG4 demonstrated binding of autoantibodies to the basement membrane of normal human skin exclusively belonging to the IgG4 subclass (L), whereas no deposits of IgG1 (I), IgG2 (J) or IgG3 (K) were observed (all magnifications, ×200).

2

IgG4 autoantibodies, in contrast to IgG1, do not fix complement to the dermal–epidermal junction. In a representative experiment, cryosections of normal human skin were incubated with serum and purified antibody preparations from patient 2 and, subsequently, treated with fresh human serum as a source of complement. Both serum (A) and purified IgG1 autoantibodies (B) fixed complement C3 at the dermal–epidermal junction in a linear fashion. In contrast, incubation of cryosections with IgG4 specific for the dermal–epidermal junction (C) or serum from a healthy control (D) does not result in C3 deposition (all magnifications, ×200).

3

IgG4 autoantibodies recruit and activate leucocytes. Sections of human skin were incubated with IgG1 and IgG4 autoantibodies from a BP patient, as well as with IgG from a healthy control. Subsequent addition of leucocytes from healthy donors leads to leucocyte attachment at the dermal–epidermal junction in sections treated with patient's IgG1 (A) and IgG4 autoantibodies (B), but not in sections incubated with control IgG (C). Activation of leucocytes, as revealed by the reduction of nitro blue tetrazolium (NBT) to formazan (dark precipitates), is induced by purified IgG1 (D) and IgG4 autoantibodies (E), but not by control IgG (F) (all magnifications, ×400). (G) Cryosections of human skin were treated with IgG1 (n = 3) and IgG4 (n = 5) antibodies (four sections/antibody preparation). Subsequently, leucocytes from healthy donors were incubated for 90 min with the cryosections. Deposition of formazan is represented as means ± SEM of the percent of the total length of the dermal–epidermal junction (DEJ) for each section.

4

IgG4 autoantibodies induce dermal–epidermal separation in sections of human skin. Results of a representative experiment show that dermal–epidermal separation in sections of normal human skin is induced by serum (A), IgG1 (B) and IgG4 autoantibodies (C) from patient 1. Serum antibodies from a healthy control (D) do not induce sub-epidermal splits (all magnifications, ×200).

5

A blocking monoclonal antibody against CD16 significantly inhibits the autoantibody-induced dermal–epidermal separation. Cryo-sections of human skin were incubated with IgG, IgG1 and IgG4 purified from BP patients 1 and 2 (n = 4 sections/antibody preparation). Subsequently, granulocytes were treated for 10 min at room temperature with 3G8 or a control antibody prior to incubation with skin sections. Dermal–epidermal separation is represented as means ± SEM of the percent of the total length of the dermal–epidermal junction (DEJ) for each section.

6

IgG4 autoantibodies show a lower pathogenic capacity compared with IgG1. In the upper panel, cryosections of human skin were incubated with IgG1 or IgG4 autoantibodies purified from patient 2, both adjusted at an end-point titer of 1:80 by IF microscopy. Subsequent addition of leucocytes leads to blister formation in cryosections treated with IgG1 autoantibodies (A). In contrast, IgG4 autoantibodies (B) fail to recruit leucocytes to the dermal–epidermal junction and to induce dermal–epidermal separation (magnification, ×400). The lower panel (C) shows the extent of dermal–epidermal separation after incubating the cryosections with IgG1 and IgG4 preparations from two BP patients (n = 4 sections/antibody preparation). Dermal–epidermal separation is represented as means ± SEM of the percent of the total length of the dermal–epidermal junction (DEJ) for each section. The reactivity of the antibody preparations as determined by their end-point titres by immunofluorescence microscopy is represented on the X-axis.