Photo from University of Texas Southwestern Medical Center
Chengcheng “Alec” Zhang, PhD
Efforts to determine why immune checkpoint blockade is not successful in treating leukemia have resulted in a “compelling” new target to treat monocytic acute myeloid leukemia (AML), according to researchers.
They discovered that leukocyte immunoglobulin-like receptor B4 (LILRB4), a marker of monocytic leukemia, creates an immunosuppressive microenvironment by mediating T-cell suppression.
Using a mouse model and human cells, the research team showed that LILRB4 supports tumor infiltration into tissues and suppresses T-cell activity through a signaling pathway involving APOE, LILRB4, SHP-2, uPAR, and ARG1.
Senior author Chengcheng “Alec” Zhang, PhD, of the University of Texas Southwestern Medical Center in Dallas, and his colleagues reported their findings in Nature.
The team first compared surface expression of LILRB4 on normal monocytes and neoplastic monocytes from 105 AML patient samples. They observed that LILRB4 levels were higher on monocytic AML cells than on normal monocytes.
The researchers then tested whether LILRB4 expressed on AML cells suppressed T cells. They cultured LILRB4-positive leukemia cells, LILRB4-negative leukemia cells, and normal hematopoietic cells with either autologous T cells or T cells from healthy donors. The team determined that only LILRB4-positive monocytic AML cells substantially suppressed T-cell proliferation.
When the researchers knocked out LILRB4, the ability of AML cells to suppress T cells was reduced and could be restored with forced expression of wild-type LILRB4. Additionally, LILRB4-mediated T-cell inhibition could be reversed by LILRB4-blocking antibodies.
The team then used a humanized mouse xenograft model and an immunocompetent mouse model to investigate LILRB4 function in immune checkpoint blockade. Blocking LILRB4 lowered tumor burden and prolonged survival in the mice.
The researchers performed numerous in vitro and in vivo experiments and observed that antibody blockade of LILRB4 shrank tumors and decreased leukemic infiltration into internal organs, including the bone marrow, liver, and brain.
And so the team hypothesized that LILRB4 promotes leukemia infiltration in addition to inhibiting T cells.
To test the hypothesis, they performed transendothelial migration and homing assays and monitored leukemia infiltration relative to LILRB4 expression on leukemia cells. They observed that LILRB4-mediated migration enhanced extramedullary infiltration of monocytic AML cells, thus contributing to immune evasion.
The researchers also found that APOE protein activated the immune inhibitory receptor LILRB4.
To ascertain whether suppression of T cells by LILRB4 depends on APOE, the team co-cultured T cells with control or human AML cells with APOE knocked out.
Through a series of experiments, they determined that APOE is an extracellular binding protein of LILRB4 and that APOE activates LILRB4 to support T-cell proliferation and AML cell migration.
The researchers believe that targeting LILRB4 may have minimal toxicity. This is because LILRB4 expression on normal monocytic cells is limited, LILRB4 signaling may differ in leukemia cells, and LILRB4 blockade did not significantly interfere with normal hematopoietic function.
Dr. Zhang anticipates that if the preclinical studies go well, clinical trials could begin as early as next year.
The University of Texas System has exclusively licensed LILRB4-related patent applications to California-based Immune-Onc Therapeutics Inc., which contributed to the research and is conducting preclinical studies.
Dr. Zhang and another author are scientific advisory board members with Immune-Onc Therapeutics. Two other authors are employees of and hold equities in Immune-Onc Therapeutics.
The researchers received additional funding for this work from the National Cancer Institute, Leukemia & Lymphoma Society, the March of Dimes, the Cancer Prevention and Research Institute of Texas, the Robert A. Welch Foundation, the National Natural Science Foundation of China, the National Basic Research Program of China, and the China Scholarship Council.