Treatment with atacicept enhances neuronal cell death in a rat model of optic neuritis

The number 13 of the family: a proliferation inducing ligand

The TNF superfamily member a proliferation inducing ligand (APRIL, TNFSF13) plays a late role in humoral immunity at the level of antibody-producing plasmocytes. The recent characterization of the first immunodeficient patient with an inactivating mutation in the APRIL gene provided the last piece of functional data lacking in the human system. Based on this function, APRIL has been considered as a valuable target to dampen unwanted antibody production. After reviewing the late data acquired on the physiological function of APRIL in humoral immunity, we will here review the state of the art regarding APRIL targeting in autoimmune diseases.

A proliferation-inducing ligand-mediated anti-inflammatory response of astrocytes in multiple sclerosis

OBJECTIVE

The two related tumor necrosis factor members a proliferation-inducing ligand (APRIL) and B-cell activation factor (BAFF) are currently targeted in autoimmune diseases as B-cell regulators. In multiple sclerosis (MS), combined APRIL/BAFF blockade led to unexpected exacerbated inflammation in the central nervous system (CNS) of patients. Here, we investigate the role of the APRIL/BAFF axis in the CNS.

METHODS

APRIL expression was analyzed in MS lesions by immunohistochemistry. The in vivo role of APRIL was assessed in the murine MS model, experimental autoimmune encephalitis (EAE). Functional in vitro studies were performed with human and mouse astrocytes.

RESULTS

APRIL was expressed in lesions from EAE. In its absence, the disease was worst. Lesions from MS patients also showed APRIL expression upon infiltration of macrophages. Notably, all the APRIL secreted by these macrophages specifically targeted astrocytes. The upregulation of chondroitin sulfate proteoglycan, sometimes bearing chondroitin sulfate of type E sugar moieties, binding APRIL, in reactive astrocytes explained the latter selectivity. Astrocytes responded to APRIL by producing a sufficient amount of IL-10 to dampen antigen-specific T-cell proliferation and pathogenic cytokine secretion. Finally, an intraspinal delivery of recombinant APRIL before disease onset, shortly reduced EAE symptoms. Repeated intravenous injections of recombinant APRIL before and even at disease onset also had an effect.

INTERPRETATION

Our data show that APRIL mediates an anti-inflammatory response from astrocytes in MS lesions. This protective activity is not shared with BAFF. ANN NEUROL 2019;85:406-420.

Profile of atacicept and its potential in the treatment of systemic lupus erythematosus

The importance of B cell activating factors in the generation of autoantibodies in patients with systemic lupus erythematosus (SLE) is now recognized. The two key factors, known as BAFF and APRIL, produced by a variety of cells including monocytes, dendritic cells and T cells, also help to regulate B cell maturation, function and survival. Biologic agents that block these factors have now been developed and tried out in large scale clinical trials in SLE patients. Benlysta which blocks BAFF has met some of its end points in clinical trials and is approved for use in patients with skin and joint disease who have failed conventional drugs. In contrast, clinical trials using atacicept which blocks both BAFF and APRIL have been more challenging to interpret. An early study in lupus nephritis was, mistakenly, abandoned due to serious infections thought to be linked to the biologic when in fact the dramatic fall in the immunoglobulin levels took place when the patients were given mycophenolate, prior to the introduction of the atacicept. Likewise the higher dose arm (150 mgm) of a flare prevention study was terminated prematurely when 2 deaths occurred. However, the mortality rate in this study was identical to that seen in the Benlysta studies and a post hoc analysis found a highly significant benefit for the 150mgm arm compared to the lower dose (75 mgm) and placebo arms. Other trials with both Benlysta and atacicept are on-going.