Using the K/BxN mouse model of endogenous, chronic, rheumatoid arthritis for the evaluation of potential immunoglobulin-based therapeutic agents, including IVIg and Fc-μTP-L309C, a recombinant IgG1 Fc hexamer

Plasma derived IgG is a major replacement therapy for primary immunodeficiency [1‐3] and is a first line treatment for diseases such as immune thrombocytopenia (ITP) [4‐9], chronic inflammatory demyelinating polyneuropathy (CIDP) [10] and Kawasaki disease [11, 12]. Additionally, it is used to treat a variety of other autoimmune and inflammatory diseases and neurologic disorders in combination with other therapies or when traditional therapies fail [13‐21]. Although both F(ab’)₂- and Fc-dependent mechanisms have been suggested to be involved in the anti-inflammatory effects of this therapy, research in the field has emphasized that the IgG Fc fragment is crucial for its immunomodulatory properties [22]. Some of the proposed Fc-dependent mechanisms include the blockade of activating Fcγ receptors (FcγRs) [23], the requirement for the neonatal Fc receptor (FcRn) [24, 25], the expansion of regulatory T cell populations [26], the upregulation of the inhibitory receptor FcγRIIB [27] and the modulation of dendritic cell activity [28, 29].

Today, plasma derived IgG is pooled from the blood of thousands of donors and manufactured via chromatographic processes to formulate a highly purified, polyclonal IgG product that is suitable for i.v. (intravenous immunoglobulin, IVIg) or s.c. (subcutaneous immunoglobulin, SCIg) applications [30, 31]. Even though the use of SCIg has increased the patient convenience associated with IVIg treatment, its manufacture still requires highly specialized production facilities with a focus on pathogen safety [31, 32]. Moreover, its supply is dependent on the availability and the collection of human plasma and it is subject to some natural variability. These challenges associated with growing product demand, production and availability have provided incentives to develop various Fc constructs as potential alternatives to IVIg/SCIg for diseases where its mechanism has been suggested to be Fc-dependent [22‐29].

Various Fc multimers have been produced that show enhanced efficacy compared to IVIg for amelioration of disease in animal models, such as ITP and arthritis [33‐41]. In a recently published paper, we reported a recombinant human IgG1 Fc, Fc-μTP-L309C, which was produced by fusing the 18 aa IgM tail-piece to the C-terminus of a variant human IgG1 Fc with a point mutation at position 309 [38]. This point mutation facilitates the stabilized hexamerization of this molecule through the formation of disulphide bonds and distinguishes this recombinant hexamer from others reported [38]. We showed that Fc-μTP-L309C has high binding avidity for Fc receptors and could suppress acute collagen antibody- and chronic collagen-induced arthritis and ameliorate ITP in mouse models when given therapeutically at 10-fold lower doses than IVIg [38].

With the limited amount of studies performed on the therapeutic efficacy of IVIg in rheumatoid arthritis (RA) [20, 42‐47], we decided to examine whether IVIg could ameliorate arthritis in an endogenous, chronic mouse model of RA. The K/BxN serum-transfer model is an established mouse model of RA that recapitulates the effector phase of the disease and has been examined for potential treatment efficacy of IVIg by a number of investigators [48‐51]. However, K/BxN mice, themselves, that spontaneously generate a true autoimmune-mediated RA, have never been used as a therapeutic tool to investigate the efficacy of RA therapeutic agents, including IVIg.

K/BxN mice are generated using KRN mice expressing a T-cell receptor (TCR) transgene for glucose-6-phosphate isomerase (G6PI) peptide in the context of IAg7 MHC II [52]. To generate K/BxN mice, KRN are bred with NOD/Lt mice expressing the MHC II haplotype IAg7 that is required to interact with the TCR transgene. This mating results in the generation of a true autoantibody to G6PI [52‐55]. G6PI is present on the articular cartilage and thus, immune complexes form to drive the activation of various immune cells [52‐55]. Since the sera of K/BxN mice contain pathogenic autoantibodies to G6PI, it can be transferred into naïve mice and arthritic manifestations occur a few days to weeks later [48‐51]. However, serum recipients develop arthritis in the absence of the adaptive immune system, which poses serious limitations for examining the intricacies of the human disease. Thus, instead of a serum-transfer model, we used the endogenous K/BxN mice to better recapitulate the autoimmune-mediated RA; the resulting immune-mediated mechanism would include the influence of T-cells, antigen presenting cells and B-cells; a true autoimmune condition. Because no one has previously shown that IVIg could ameliorate the endogenous chronic arthritis in this mouse model, we used a dose escalation and frequency of dosing approach to determine if IVIg had any efficacy in this RA model. We also investigated the therapeutic efficacy of SCIg and Fc-μTP-L309C in this same model to determine efficacy, if any, and whether any of these agents could serve as a potential therapeutic alternative for the treatment of RA.

The value of IVIg in the treatment of RA has not been extensively evaluated. The human trials that have been done to evaluate this were small and not well controlled, and the results were equivocal [20, 42‐47]. The patient cohorts for these studies were small, with some studies having less than 20 patients enrolled [42, 43]. A high-dose IVIg protocol of 1–2 g/kg of IVIg per month for a minimum of 6 months is critical for effectively treating systemic autoimmune diseases [59]. However, these aforementioned studies used lower than immunomodulatory doses of IVIg, used different doses and dosing schedules without any consistent long-term follow-up [42‐47]. In mouse models of RA, only collagen-induced and serum-transfer models have been used to evaluate therapeutic efficacy of immunoglobulin-based agents [38, 60, 61]. Given the lack of understanding we have about the therapeutic efficacy of IVIg in RA, we wanted to determine if the K/BxN mouse model of endogenous, chronic rheumatoid arthritis could be used to evaluate potential therapeutic treatments.

We first investigated whether IVIg could treat autoimmune-mediated arthritis in the K/BxN mouse model [56‐58]. Other researchers have used the K/BxN serum transfer model to study arthritis whereby the inflammatory response in the serum recipients happens in the absence of the adaptive immune system [48‐51]. Although this model is a useful tool to understand how autoantibodies drive the progression of arthritis by interacting with downstream components of the innate immune system, there are some major differences between the serum transfer model and the human disease. These include differences in antibody specificities and function, and differences in immune cells that drive pathogenesis [53, 55]. Considering this, we decided to treat K/BxN mice with severe arthritis endogenously because we, and others, believe that the immune cells involved in driving the inflammatory response in K/BxN mice more tightly mimic those experienced in the human seropositive disease [52, 57, 58].

We are the first to use immunoglobulin-based agents to try and treat the spontaneous generation of RA in the K/BxN mouse model. Therefore, we first used a multiple dose and frequency of dosing approach to determine if there was any effect on the RA and, if so, the optimal treatment regimen where IVIg was effective. We used an established clinical scoring system to determine when the mice had reached a high level of arthritis, which resulted in a clinical score of 10–12 per mouse. Upon reaching a high clinical score, we then treated these mice with 4 g/kg, 2 g/kg and 1 g/kg of IVIg, 2 times per week. With this dosing scheme, we did not observe any decrease in clinical scores or amelioration of arthritis (data not shown). Next, we treated these mice with the same doses of IVIg, administered every other day. With this dosing scheme, we found that the mice that were given 2 g/kg and 4 g/kg doses showed a similar, significant decrease in their clinical scores and in their paw measurements in comparison to mice given HSA. This is something that has not been shown before in this mouse model of arthritis, however it is comparable to results found in the K/BxN serum-transfer model [40, 50] and in the more commonly known arthritis models such as the collagen-antibody-induced arthritis (CAIA) and the collagen-induced arthritis (CIA) models [38], demonstrating the efficacy of IVIg in mouse models of arthritis [38, 48, 49, 51, 62, 63].

Upon establishing the efficacy of IVIg in the K/BxN mouse model, we wanted to compare its effects with those of Fc-μTP-L309C and SCIg. Both i.v. and i.p. injection of IVIg have been used previously without any evidence of a difference in efficacy. Because i.p. is easier and more volume can be injected, we used i.​p.​in our studies. We observed that SCIg and IVIg were equally effective at ameliorating endogenous arthritis and that Fc-μTP-L309C was more effective than both SCIg and IVIg at 10-fold lower doses. It should be noted that Fc-μTP-L309C also exhibited higher therapeutic efficacy when given intraperitoneally in both the CIA and in the CAIA mouse models of arthritis [38]. In our case, to better insure we would see an effect, we used high doses of Fc-μTP-L309C, due to the higher frequency of dosing, and was based on the serum half-life of Fc-μTP-L309C to be very short [38]. Additionally, it has been proposed that subcutaneous absorption of biotherapeutics is relatively slow and mostly incomplete [64]. These factors also pose potential issues in developing dosing schemes to test the efficacy of Fc-μTP-L309C to treat human diseases.

Lastly, as a complementary study to our work in the endogenous model, we wanted to know if IVIg, SCIg or Fc-μTP-L309C had any prophylactic activity. We used both the endogenous model and the serum transfer model to investigate this. For the endogenous model we treated mice with either IVIg, SCIg, or Fc-μTP-L309C at weaning age before arthritis had developed. In both models, we found that both IVIg and SCIg prevented disease onset equally in these mice with the exception of a small rebound at the start of the treatment cycle in the serum transfer model. Fc-μTP-L309C however, exhibited complete prevention without any rebound at the start of the treatment cycle.

In summary, we are the first group to show that K/BxN mice, a model for human RA, having endogenous, chronic, severe arthritis can be effectively treated with IVIg, SCIg and with a recombinant protein, Fc-μTP-L309C. This model is a close representation of human autoimmune RA that is superior to the passive antibody-induced serum-transfer model of arthritis, which models only the effector phase of arthritis and does not involve the adaptive immune system. Our work demonstrates the utility of using the K/BxN mouse for evaluating potential therapeutic agents for treatment of RA. Although the efficacy of both IVIg and recombinant Fcs have yet to be confirmed in human studies, our results show that perhaps using higher doses and more frequent dosing in human RA could have benefit. Perhaps immunoglobulin-based therapies would show efficacy if used to treat certain subgroups of patients, or to treat patients with comorbidities that preclude them from using first line therapies, or to treat patients in whom other therapies are contraindicated. Importantly, the therapeutic effects of both IVIg and Fc-μTP-L309C could be achieved with s.c. routes of administration. The s.c. administration of IgG products has become increasingly attractive in recent years due to patient convenience (home administration) and better systemic tolerability. Considering that lower doses are needed for Fc-μTP-L309C to exhibit therapeutic efficacy in comparison to IVIg with the s.c. route of administration, Fc-μTP-L309C, given subcutaneously, could serve as a possible replacement therapy for IVIg/ SCIg in certain autoimmune diseases.