DPPX antibody-associated encephalitis: Main syndrome and antibody effects

Abstract

Objective: To report the main syndrome of dipeptidyl-peptidase-like protein 6 (DPPX) antibody-associated encephalitis, immunoglobulin G (IgG) subclass, and the antibody effects on DPPX/Kv4.2 potassium channels.

Methods: A retrospective analysis of new patients and cases reported since 2013 was performed. IgG subclass and effects of antibodies on cultured neurons were determined with described techniques.

Results: Nine new patients were identified (median age 57 years, range 36-69 years). All developed severe prodromal weight loss or diarrhea followed by cognitive dysfunction (9), memory deficits (5), CNS hyperexcitability (8; hyperekplexia, myoclonus, tremor, or seizures), or brainstem or cerebellar dysfunction (7). The peak of the disease was reached 8 months (range 1-54 months) after onset. All patients had both IgG4 and IgG1 DPPX antibodies. In cultured neurons, the antibodies caused a decrease of DPPX clusters and Kv4.2 protein that was reversible on removal of the antibodies. Considering the current series and previously reported cases (total 39), 67% developed the triad: weight loss (median 20 kg; range 8-53 kg)/gastrointestinal symptoms, cognitive-mental dysfunction, and CNS hyperexcitability. Outcome was available from 35 patients (8 not treated with immunotherapy): 60% had substantial or moderate improvement, 23% had no improvement (most of them not treated), and 17% died. Relapses occurred in 8 of 35 patients (23%) and were responsive to immunotherapy.

Conclusions: DPPX antibodies are predominantly IgG1 and IgG4 and associate with cognitive-mental deficits and symptoms of CNS hyperexcitability that are usually preceded by diarrhea, other gastrointestinal symptoms, and weight loss. The disorder is responsive to immunotherapy, and this is supported by the reversibility of the antibody effects in cultured neurons.

Figure 1. Demonstration of DPPX antibodies and IgG subclasses in a cell-based assay

(A) Serum of a representative case (patient 4) showing reactivity (green, A.a) with human embryonic kidney (HEK) cells expressing dipeptidyl-peptidase–like protein 6 (DPPX). The reactivity of a commercial antibody against DPPX (red, A.b) colocalizes with that of the patient's serum (yellow, A.c). Note that a control serum is negative (A.d–A.f). (B) Determination of immunoglobulin G (IgG) antibody subclasses in 2 cases (patients 4 and 1). Patients' antibodies bound to HEK cells expressing DPPX are demonstrated with secondary anti-human antibodies specific for the indicated subclasses. Patient 4 has DPXX antibodies of the IgG1, IgG2, and IgG4 subclasses (B.a–B.c), whereas patient 1 has IgG1 and IgG4 subclass antibodies (B.d–B.f). Nuclei counterstained with 4′, 6-diamino-2-phenylindole (A and B). Scale bars = 10 μm.

Figure 2. Effects of patients' antibodies on DPPX and Kv4.2 are reversible

(A–D) Clusters of dipeptidyl-peptidase–like protein 6 (DPPX) in cultured neurons incubated for 3 days with pooled control immunoglobulin G (IgG), pooled patients’ (cases 4 and 7) IgG without recovery, and pooled patients’ (cases 4 and 7) IgG for 3 days followed by 4 or 7 days of recovery (fresh media without antibodies). Scale bars = 10 μm. Boxed dendrites are shown at higher magnification below (×100/1.3 numerical aperture oil objective). Note the reduction of clusters of DPPX caused by 3 days’ exposure to patients' antibodies and the progressive restoration of the density of DPPX clusters after neurons are allowed to recover for 4 to 7 days. (E) Graphic representation of the density of DPPX clusters (median with interquartile range) after treatment for 3 days with control or patients' IgG and after 4 to 7 days of recovery (4 independent experiments, 15 neurons per experiment in each condition). (F) An immunoblot of biotinylated surface proteins of hippocampal neurons treated with pooled control IgG and pooled patients’ IgG for 3 days without or with recovery for 4 to 7 days. The protein bands are demonstrated with commercial antibodies specific for DPPX and Kv4.2; NR1 and transferrin receptor (TfR) are used as loading controls. Note that patients' antibodies cause a substantial decrease of levels of DPPX and Kv4.2 after a 3-day incubation and that the levels of cell-surface proteins are progressively restored after 4 to 7 days. (G) Quantitative densitometry analysis of panel F; the data were normalized to the values of control IgG and are represented as median with SEM of 3 independent experiments (for each experiment, immunoblots were repeated, n = 6). All statistical analyses: Kruskal-Wallis test followed by Dunn test, *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.