General biology of BLyS
BLyS (also known as BAFF) is a cytokine that is essential for survival of most B cells beyond the transitional 1 stage [104
]. BLyS binds to three receptors: BCMA, TACI, and BR3 (also known as BAFFR). Two of these receptors (BCMA and TACI), but not the third (BR3), also bind APRIL, a cytokine closely related to BLyS that shares some, but not all, of its biological properties. Whereas survival of plasma cells, which express TACI and BCMA, is supported by either BLyS or APRIL, the survival of preplasmablast mature B cells, which express much BR3 but little TACI and essentially no BCMA, is supported only by BLyS. Of note, memory B cells appear to be independent of both BLyS and APRIL.
In addition to affecting B-cell survival/function, BLyS can affect other cell types that express BLyS receptors. Specifically, BR3 is expressed on T cells, although its role in T-cell signaling/stimulation is controversial [104
]. Moreover, dendritic cells also express BLyS receptors, and BLyS-stimulated dendritic cells upregulate co-stimulatory molecules and produce inflammatory cytokines and chemokines such as IL-1, IL-6, CCL2, and CCL5.
BLyS in murine systemic lupus erythematosus
There is an irrefutable link between BLyS and murine SLE. Constitutive overexpression of BLyS in BLyS-transgenic nonautoimmune-prone mice leads to SLE-like features, including elevated levels of multiple autoantibodies (including anti-dsDNA), circulating immune complexes, and glomerular immunoglobulin deposition [105
]. Moreover, BLyS overexpression accelerates the development of SLE-like features in mice that bear an autoimmune diathesis but otherwise do not develop overt SLE [106
]. On the other hand, SLE-prone NZM2328 mice genetically deficient in BLyS are largely spared from overt disease (severe proteinuria and premature death), although the lifelong absence of BLyS does not protect them from ultimately developing serological autoimmunity and renal pathology [107
]. Most importantly, treatment of BWF mice, (NZMxBXSB)F1 mice, MRL/lpr mice, or NZM2410 mice with either TACI-Ig (which neutralizes both BLyS and APRIL) or BR3-Ig (which selectively neutralizes BLyS) is effective at preventing clinical disease and ameliorating renal injury [108
]. Intriguingly, IFNα-driven exaggerated disease in several SLE-prone mouse strains is associated with increases in serum BLyS levels [59
], and IFNα-driven exaggerated disease is completely blocked in BLyS-deficient NZM2328 mice [109
], indicating that BLyS is a vital contributor to the IFNα-driven pathogenic pathway in SLE.
BLyS in human systemic lupus erythematosus
Not only is BLyS associated with murine SLE, but is also associated with human SLE. Circulating BLyS levels are elevated in as many as 50% of SLE patients [110
], and disease activity correlates with blood leukocyte expression of BLyS mRNA [111
]. Aberrant expression of BLyS on multiple immune cells has been observed in SLE. Although BLyS is largely expressed in myeloid lineage cells, activated B cells can also express both BLyS and APRIL. In SLE patients, B cells and plasma cells express high levels of BLyS and APRIL mRNA, which correlate with disease activity and levels of anti-dsDNA antibodies [112
In addition to the biologic studies summarized above, investigations into genetic susceptibility loci in SLE have revealed associations between polymorphisms in BLyS and APRIL genes with human SLE [113
]. In light of the results from murine SLE models, from ex vivo
studies of human SLE leukocytes, from BLyS serum analyses, and from the genetics of SLE, BLyS has emerged as a highly attractive cytokine target in SLE.
Therapeutic blockade of BLyS in human systemic lupus erythematosus
The greatest experience to date with BLyS antagonists (Table ) has accrued with belimumab, a fully human IgG1λ
mAb that binds and neutralizes soluble BLyS [115
]. Belimumab was shown to be safe in a randomized, double-blind, placebo-controlled phase I trial of SLE, in which the prevalence of adverse events was no different between belimumab-treated and placebo-treated patients [116
]. Of note, only modest reductions in peripheral blood B cells were observed among belimumab-treated patients. No clinical efficacy was demonstrated in this phase I trial, but the small number of patients (n
= 70) and the very brief treatment schedules (single infusion or two infusions 3 weeks apart) and follow-up period (12 weeks after final infusion) precluded demonstration of clinical benefit.
Disappointingly, the subsequent phase II trial (n
= 449) failed to meet its co-primary endpoints [117
]. Extensive post hoc
analysis, however, led to a novel composite index of clinical response (SLE responder index) [118
] and demonstrated significantly increased clinical response among belimumab-treated patients at 52 weeks among the patients who were seropositive (ANA titer ≥1:80 and/or positive for anti-dsDNA antibodies) at entry. Using this novel SLE responder index, two separate large randomized, double-blind, placebo-controlled phase III trials (n
= 865 and n
= 819, respectively) of belimumab in seropositive SLE patients each met their primary endpoints (increased percentage of responders at 52 weeks) [119
]. Importantly, analysis of the combined 1,864 SLE patients in both trials pointed to reductions in disease activity and prevention of worsening across vital internal organ systems [121
]. Although questions remain regarding the durability of the clinical response, a US Food and Drug Administration advisory panel in November 2010 recommended approval for belimumab in the treatment of SLE, and final approval by the US Food and Drug Administration was given on 9 March 2011.
Concern has been raised regarding the ostensibly lower clinical efficacy of belimumab in human SLE in comparison with the ostensibly more robust clinical efficacy of BLyS antagonists in murine SLE. This difference in clinical response between human and murine SLE may be more apparent than real. First, the clinical response in the phase III human trials was based on a composite of several instruments each rooted in multiple organ systems [118
]. In contrast, clinical response in the murine trials was simply the absence of pre-moribund proteinuria and death. (Belimumab in all likelihood will prove to be very effective in preventing pre-moribund proteinuria and death; however, it will take many years of follow-up to formally prove this point.) Second, human SLE patients received standard-of-care therapy in addition to belimumab in the human clinical trials, whereas murine SLE patients in the murine clinical trials received no treatment other than a BLyS antagonist. (Human SLE patients treated with belimumab alone would undoubtedly do much better than untreated patients, but such a human trial would be entirely unethical.)
In addition to belimumab, several other BLyS antagonists are undergoing clinical evaluation in SLE. The one furthest advanced in clinical evaluation is atacicept, a fusion protein between one of the BLyS receptors (TACI) and the Fc portion of IgG. Favorable safety and tolerability were demonstrated in a randomized, double-blind, placebo-controlled phase I trial (n
= 49) [122
]. Dose-dependent reductions in peripheral blood B cells and in circulating immunoglobulin levels were noted, but clinical efficacy could not be demonstrated due to the limited treatment and limited follow-up period. Of concern, an increased risk of severe infections was observed in a subsequent trial involving patients with SLE nephritis who were concurrently taking mycophenolate mofetil and corticosteroids (NCT00573157). As a consequence, this trial was prematurely terminated. Nevertheless, a separate phase II/III trial of atacicept in SLE has recently been initiated (NCT00624338). Whether atacicept achieves clinical success from efficacy and safety standpoints remains to be determined.
A third BLyS antagonist being tested in clinical trials is A-623 (previously known as AMG 623), a fusion between the Fc portion of IgG and a peptide sequence selected for its ability to bind with high affinity to BLyS. In a double-blind, placebo-controlled phase I trial, SLE patients received a single dose (n
= 54) or four weekly doses (n
= 63) of escalating doses of AMG 623 or matching placebo [123
]. A dose-independent decrease in naive and total peripheral blood B cells was accompanied by an increase in memory B cells - an observation that has now also been made in patients treated with atacicept or belimumab [122
]. Clinical responses were not reported, so the relevance of the disparate changes among B-cell subsets to clinical parameters remains unknown. A phase II trial of A-623 in SLE had been initiated but was suspended due to 'structural failure identified in some product vials', but the trial was recently resumed (NCT01162681).
A fourth BLyS antagonist in clinical development for SLE is LY2127399, a mAb that binds both soluble and membrane BLyS [125
]. Two phase III trials in SLE are planned (NCT01205438 and NCT01196091), but neither has yet started recruiting patients. It remains to be determined whether neutralization of soluble plus membrane BLyS (as with LY2127399) will have greater therapeutic efficacy than neutralization of soluble BLyS alone (as with belimumab).