Researchers headed by a team at Seattle Children’s Research Institute have shown for the first time that engineered human plasma B cells can be used to treat a disease—more specifically leukemia—in a humanized mouse model. The results mark a key step in the realization of engineered plasma cells (ePCs) as therapies to treat cancer, autoimmune disorders, and protein deficiency disorders.

“We hope that this proof-of-concept study is the first of many applications of engineered plasma B cells, and eventually will lead to a single-shot therapeutic,” said Richard James, PhD, at the Seattle Children’s Research Institute. “Because engineered plasma B cells can live for a very long time, greater than 10 years, they could be used as a long-term source of many biologic drugs.” James is senior author of the team’s published paper in Molecular Therapy, titled “Human plasma cells engineered to secrete bispecifics drive effective in vivo leukemia killing,” in which they stated, “These findings support further development of ePCs for use as a durable delivery system for the treatment of acute leukemias, and potentially other cancers.”

Immunotherapies such as bispecific antibodies that recruit cytotoxic T cells to kill cancer cells have contributed to improved survival rates for patients with B cell acute lymphoblastic leukemia (B-ALL), the authors wrote. Blinatumomab is a bispecific antibody that has been approved for about a decade, for treating patients with relapsed/refractory B-ALL. The authors explained, “Blinatumomab is an anti-CD19 x anti-CD3 non-immunoglobulin G-like bispecific antibody (non-IgG-like bispecific; also called a bispecific T cell engager) that received approval from the U.S. Food and Drug Administration in 2014 for the treatment of patients with relapsed/refractory B-ALL.” However, a limitation of blinatumomab therapy is that it requires continuous high-dose intravenous infusions to maintain activity. “Bispecific non-immunoglobulin therapies pose stability challenges in patients, necessitating three courses of 20-day steady-state infusion,” James said. This intensive regimen poses challenges for patients because frequent bag changes prove inconvenient, and the use of ports increases the risk of infection. The authors further poited out, “Enhanced drug delivery methods for bispecific antibodies like blinatumomab could improve patient adherence and bolster treatment efficacy.”

Work by James and others has explored using engineered plasma cells (ePCs) for long-term protein drug delivery, the team continued. Engineered B cells have been investigated in proof-of-concept studies to deliver biologic drugs to treat protein deficiency diseases, viral infections and cancer. Such cells are “uniquely suited to deliver biologics over long period” due to their long lifespan and high secretory capacity,” the team continued. “Because a subset of PCs and ePC preferentially localize to bone marrow and other tissue microenvironments where progenitor B-ALL cells reside, we predicted that ePCs could harmonize with local bispecific delivery to induce potent anti-leukemia activity.”

James added, “We think that the first application of engineered plasma B cells will be to produce drugs that are difficult for patients to use. In this study, we wanted to demonstrate proof of concept and efficacy for engineered B cell therapies.”

For their reported study the investigators developed a gene-editing strategy for generating ePCs that produce large quantities of bispecifics to target B-ALL or acute myeloid leukemia. “… we describe a homology-directed repair (HDR)-based gene editing strategy for the generation of ePC that produce large quantities of anti-CD19 x anti-CD3 or anti-CD33 x anti-CD3 non-IgG-like bispecifics to target B-ALL or acute myeloid leukemia (AML), respectively,” they wrote. The combined findings of their experiments demonstrated that ePCs secreting bispecifics can promote T cell-driven killing of primary human cells and human leukemic cell lines.

“One challenge we encountered was that the bispecific antibody used for killing tumor cells can also bind the engineered plasma B cells because they express the same target protein,” James stated. “To overcome this challenge, we deleted the protein target of the antibody, CD19, when we were making the engineered cells. We were surprised that deletion of CD19 did not hinder manufacturing of engineered plasma B cells.”

In addition, the researchers discovered that plasma cells secreting anti-CD19 bispecific antibodies elicited antitumor activity, as demonstrated with acute lymphoblastic leukemia patient-derived xenografts in immunodeficient mice co-engrafted with autologous T cells.

Notably, the steady-state concentration of anti-CD19 bispecifics in serum one month after cell delivery and tumor eradication was comparable to that observed in patients treated with continuous infusion of blinatumomab. “… we obtained in vivo serum concentrations of the bispecific surpassing that of the steady-state plasma concentration (CPss) seen in patients undergoing continuous infusion of blinatumomab,” the investigators noted.

Based on their results the researchers propose that ePC strategies could increase the functional half-life of bispecifics in patients with acute leukemias and other diseases where treatment half-life is limiting or where plasma cell local delivery could enhance therapeutic efficacy. The results also suggest that prolonged clinically relevant levels of bispecific and perhaps other biologics can be achieved via a single administration of ePCs. “The robust levels of bispecific achieved by ePCs compare favorably with those observed by other bispecific-secreting cell products, including macrophages and T cells, which did not produce detectable levels in serum after in vivo transfer,” the authors stated.

They suggest that their findings support further development of ePCs for use as a durable delivery system for the treatment of acute leukemias and potentially other cancers. “We created engineered plasma B cells capable of continuously producing bispecific antibodies throughout the treatment period after only one injection,” James noted. “These cells effectively eliminated tumors to a comparable extent as the clinical drug. The key takeaway is that engineered plasma B cells can provide long-lasting drug production in vivo.”

The team acknowledged that ePC bispecifics should be carefully evaluated for several possible toxicities if used clinically. Persistent on-target, off-tumor toxicity to normal bystander B cells is common in patients receiving B cell-targeted therapeutics. “In addition, for treatment of a B cell malignancy, it may be difficult to engineer a patient’s own B cells to be used as the therapy because there is a risk that some of the B cells will be cancerous,” James commented. “We did not test whether we can use a different person’s B cells to produce the bispecific antibody. Studies using such allogeneic products will likely need to be done before this specific therapy can be used to treat B cell cancers.”

As noted by the authors, further studies in humanized mice and in non-human primates are warranted to fully understand the activity, longevity, and tissue localization of ePCs. In the short term they plan to test whether engineered plasma B cells that produce bispecific antibodies are effective in other B cell-mediated diseases, including autoimmunity. These tests will initially be conducted in animal models. Additionally, the researchers are developing engineered plasma B cells to produce other therapeutic drugs, such as those needed in protein deficiency diseases such as hemophilia. They are exploring other potential applications of engineered B cells, including modifying other immune cells to either enhance or suppress the immune system.

In their paper the authors concluded, “Our findings suggest that ePCs may provide benefits for delivery of protein therapeutics beyond delivery of bispecifics as studied here … The potential for ePCs to persist long term and produce robust levels of exogenous protein could be a key to unlocking the therapeutic potential of biologics or therapeutic peptides limited by poor pharmacokinetics.”

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