Goal: To determine the mechanism by which IgM immunotherapy restores immune homeostasis in Type 1 Diabetes (T1D).
Background: IgM immunotherapy prevents the onset and progression of T1D.
Methods: 1) 5wks old non-obese diabetic (NOD) mice received human IgM (hIgM, 50ugs/wk) or saline, beginning at 5wks of age until 18wks. 2) For diabetes reversal, NOD mice were treated after the onset of diabetes with two doses (100ug) of either prediabetic NOD IgM or C57BL/6 IgM on Days 1 and 4, and their blood glucose monitored serially. 3) 5wks-old C57BL/6 and NOD mice, C57BL/6 and NOD VH125 mice, and humanized BLT mice received IgM (100ug igM on Day 1 followed by 50ug on Days 3,5,7 and 10) followed by spleen and bone-marrow cell harvest on Day 13. VH125 mice possess a heavy chain specific for human insulin knocked into the endogenous IgM locus. This heavy chain combines with endogenous light chains to produce insulin-reactive B lymphocytes. Humanized BLT mice are NOD/SCIDs that have been cotransplanted with human liver and thymus tissues, along with autologous CD34+ hematopoietic stem cells. Flow cytometry (FC) or Time of flight mass cytometry (CyTOF) was performed.
Results: 80% of saline-injected NOD mice became diabetic by 18-20 weeks of age (n=30). In contrast, none of the hIgM treated mice developed diabetes (n=10). Two doses of IgM (100ug) reversed hyperglycemia in new-onset diabetic mice and maintained BG<200mg/dL in 63% of mice for the entire duration of time (2 months) that they were monitored post treatment (n=11). In contrast, IgM derived from pre-diabetic NOD donors did not reverse diabetes.  IgM therapy expanded both, myeloid derived suppressor cell and peripheral regulatory T cell (Treg) populations. Thymic Tregs were also expanded in a regulatory B cell-dependent manner. IgM therapy diminished autoreactivity in NOD mice by A) reducing the percentage of marginal zone B cells, a subset associated with perpetuating autoimmunity (p<0.05); B) increasing transitional B cell proportions (p<0.01), indicating normalization of B cell homeostatic defects; and C) inhibiting plasma insulin autoantibody levels (p<0.0001). In NODVH125 mice, IgM eliminated insulin binding B cell population (p<0.0001) indicating a reduction in autoreactive B cell activation. In humanized BLT mice, hIgM therapy expanded the Helios+Foxp3+Treg population.
Conclusions: IgM therapy reverses new onset T1D by restoring immune homeostasis and diminishing autoreactivity. The clinical relevance of IgM therapy is confirmed in the humanized BLT mouse model wherein hIgM therapy induces the expansion of Tregs. This beneficial effect may be translatable to diabetic patients or islet graft recipients.