(136f) Recapitulating Tissue Microenvironments with Biomaterials to Restore Immunity

Introduction: Deficiencies in the immune system can result in complications such as infections, autoimmune conditions and cancer. We investigated if the recapitulation of the bone marrow stroma with a biomaterial scaffold can drive immune cell development by increasing (i) the number of T- and B-lymphocytes and (ii) the immune repertoire. We used hematopoietic stem cell transplantation (HSCT) an an exemplary case of profound immunodeficiency, in which the pre-transplant conditioning regimen destroys immune cells and severely compromises the adaptive arm of the immune system. Immunological complications are common in patients receiving HSCT and are the primary cause of treaatment failure.

Materials and Methods:  Injectable macroporous protein-polymer hydrogels, composed of alginate and bone-forming proteins were synthesized by low-temperature cryopolymerization. Differentiation of bone marrow isolated progenitor cells was used to assess the bioactivity of proteins. Sub-lethal and lethal irradiation in C57BL/6 mice was used to mimic immunodeficiency and myeloablative transplant conditioning respectively. Transplants were performed into the CD45.2⁺ mouse strain and monitored for donor-derived cells from congenic CD45.1⁺ B6.SJL mice. Hydrogels were injected subcutaneously to induce the development of a hematopoietic niche environment capable to generating lymphoid competent cells. The recovery of immune cells in peripheral blood was periodically monitored and sequencing of the T-cell receptor (TCR) was used to determine the diversity of the T-cell repertoire. An acute myeloid leukemia (AML) cancer model was used to test the efficacy of accelerated immune reconstitution. The generation of subset specific T-lymphocytes was investigated using the scaffold.

Results and Discussion: In vitro, the proteins within the hydrogel retained their bioactivity and differentiated Lin^-c-kit⁺Sca-1⁺ (LKS) cells into naïve CD4⁺ and CD8⁺T-cells. In vivo, scaffolds induced the formation of a subcutaneous bony nodule in mice. A histological analysis of the injected hydrogel revealed the formation of a bony nodule with bone marrow, populated with lymphoid progenitors. In sub-lethally irradiated mice, the hydrogel accelerated the reconstitution of T- and B-lymphocytes by over two orders of magnitude within 3 weeks by recruiting and driving lymphocyte differentiation of the endogenous progenitor HSCs. In lethally irradiated mice that were transplanted, the hydrogel served as a preferential site for transplanted progenitor HSC engraftment and induced their differentiation into T- and B-cells. Recovery of the adaptive immune system followed a similar trend to that of the sub-lethally irradiated mice. The hydrogel-mediated expansion of the T-competent progenitor pool resulted in an increase in the thymic output of transplanted mice and corresponded to an increase in the naïve T-cells pool. Sequencing of the TCR revealed an increase in the frequency and diversity, as measured by the recombination of the variable (V) and joining (J) segments of the TCR gene, of the T-cell repertoire, mediated by the hydrogel. When combined with an anti-leukemia vaccine, accelerated immune reconstitution prevented AML relapse in mice. When combined with low dose IL-2, the scaffold rapidly expanded regulatory T-cells in vivo.

Conclusions: The results indicate that a programmable biomaterial with biological cues can recapitulate aspects of the bone marrow stroma. By increasing the available sites of donor cell engraftment and providing the cues for lymphopoiesis, the reconstitution of adaptive immunity can be accelerated after HSCT, which can potentially decrease HSCT-associated immunological complications and improve the treatment of immunological disorders.

Acknowledgements: The work was supported by the National Institutes of Health through grants U19HL129903 and R01EB014703