Background/Purpose: We recently reported an advance in modeling the human immune system in mice, (Kalscheuer et al. Sci Transl Med 2012) involving maturation of patient immune systems from marrow hematopoietic stem cells (HSCs) in NOD/SCID/IL-2 receptor γ chain^null immunodeficient mice receiving partially HLA-matched human fetal thymic tissues to recreate the adult donor‘s functional and diverse T and B cell repertoires. These resemble the donor repertoire in tolerance of self and non-self recognition, but predominantly consist of naïve phenotype T cells. Since HSCs contain the genetic information to recreate a replica of each individual’s unique immune system, this Personalized Immune Mouse (PIM) affords a powerful and novel opportunity to recreate a functional human immune system of a patient with a systemic autoimmune disease, generated through thymic (for T cells) and bone marrow (for B cells) selection de novo from adult HSCs.

Methods: We explored the feasibility of creating replicas of RA patient immune systems to study early events during formation of their immune system. 120 RA patients meeting classification criteria were sequence-based typed for HLA-ABC and DRB1alleles to identify those who matched similarly HLA-typed available fetal thymic tissue by at least one HLA-A allele and one shared epitope-encoding HLA-DRB1 allele. After informed consent, 15-25 mls of RA donor marrow was aspirated yielding ~2×10⁶ CD34+ cells isolated by magnetic beads. ~2.5×10⁵HSCs were injected intraosseously into each mouse in a cohort of replicates irradiated with 2Gy, that had the T cell-depleted thymic fragment implanted under the renal capsule.

Results: To date, HSC preparations from 3 RA patients, aged <34 yrs, were studied. The third donor generated an RA immune system that at 12 weeks resembled those established from healthy controls in having 12-20 % of circulating hCD45+ cells of human origin, of which 2/3 were hCD19 and 1/3 hCD3. At week 14 the thymic grafts contained 2-9×10⁶human thymocytes, ~70% double positive. However, in marked contrast to reconstitutions with healthy control HSCs, where 80-90% of circulating CD4 T cells typically exhibit the CD45RA+CD45RO- naïve phenotype, in RA-PIM 5-25% of the CD4 T cells expressed HLA-DR and from 50 to 90% of circulating CD4 T cells had lost expression of CD45RA, reflecting activation and memory-effector differentiation. By week 14, 5-30% of bone marrow and splenic T cells were terminally differentiated to CD28-CD45RA+ and similar proportions expressed CD57, CD69, and HLA-DR, T cell phenotypes long recognized to occur in RA patients. The proportion of immature RA-PIM marrow B cells was also reduced compared to healthy controls, reflecting enhanced differentiation to naïve and transitional B cells.

Conclusion: In the initial weeks of immune cell development from HSC, replicas of RA immune systems exhibit marked CD4 T cell activation and T and B cell differentiation, suggesting their intrinsic hyperactivity compared to quiescent cell populations comparably generated from healthy controls.  These results suggest that there may be genetically-predetermined defects in the selection or regulation of lymphocytes in RA that can be replicated and analyzed in PIM.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-ra-immune-system-recreated-in-immunodeficient-mice-by-thymic-differentiation-from-ra-patients-hematopoietic-stem-cells-exhibits-marked-cd4-t-cell-activation-and-differentiation/