LRP1-Deficient T Cells Protect Against Acute Graft Versus Host Disease in MHC-Mismatched Mice

Hematopoietic stem cell transplantation (HSCT) is the treatment of choice for many types of malignant and non-malignant hematological and autoimmune diseases. Graft-versus-host disease (GVHD) is a complication that occurs in up to 50% of patients following allogeneic hematopoietic stem cell transplantation (allo-HSCT), where donor T- and B cells derived from the graft recognize and attack host antigens. Both acute and chronic GVHD continue to be a major cause of morbidity and mortality after allo-HSCT in cancer patients. For many years, there have been few effective treatment options for GVHD patients. Corticosteroids remain the first-line systemic treatment, in addition to other anti-inflammatory therapies. Unfortunately, such systemic treatments are often associated with adverse side-effects. Currently there is only one FDA-approved targeted treatment against acute GVHD, Ruxolitinib - a JAK1/2 inhibitor. However Ruxolitinib is also limited by the potential for toxicities, such as thrombocytopenia. Thus, there is clearly an urgent clinical need to optimize current therapies and to identify new candidates for targeted treatments against GVHD.

Low density lipoprotein receptor-related protein 1 (LRP1) has been recently reported to play an important role in T cell proliferation. When LRP1-expressing T cells are co-cultured with neutrophils displaying membrane bound proteinase 3 (mPR3), it results in an inhibition of T cell proliferation. Blocking the receptor with anti-LRP1 antibody leads to a reversal of this inhibition, allowing for a "rescue" of proliferation. To investigate the capacity of LRP1 to regulate T cell proliferation in vivo we established a conditional T cell specific deletion of LRP1 (LRP1 ^fx/fx Lck^Cre+) in C57BL6 mice. To induce T cell activation and proliferation in vivo, an MHC-mismatched mouse model of allo-HSCT was utilized. Recipient BALB/c mice were exposed to high dose radiation (800Gy) to achieve complete myeloablation and then received a mix of 10x10⁶ splenocytes from LRP1 KO mice (LRP1 ^fx/fx Lck^Cre+) and 5x10⁶ bone marrow (BM) cells from control mice (LRP1 ^fx/fx). As controls, two other groups of lethally-irradiated BALB/c mice were injected with either splenocytes from LRP1 ^fx/fx mice (floxed-only control) and control BM cells or a mix of Lck^Cre+ splenocytes (Lck^Cre+ -only) and control BM cells. Expectedly, BALB/c mice from the control group that received LRP1 ^fx/fx T cells developed significant acute GVHD including weight loss, hunched posture and reduced mobility, skin scaling, fur loss, which was eventually lethal. Surprisingly, recipient mice transplanted with splenocytes containing LRP1-deficient T cells had significantly lower GVHD clinical score (mean score is 7.78 vs. 0.5, p^∗=0.01), resulting in survival of 100% of mice (p^∗∗∗=0.0006). The peripheral blood analysis of BALB/c mice from the LRP1 KO group showed that a number of mice established a mixed donor chimerism of 50% C57BL6 donor (H2Db+) and 50% of BALB/c recipient (H2Db-) T cells. Histology of GVHD-target organs indicated that mixed chimeric mice had less tissue damage compared to fully chimeric mice (mean colon tissue damage score is 0.54 vs. 1.77).

To characterize the effect of loss of LRP1 on T cells, we evaluated the phenotype of LRP1 KO T cells and found no significant differences in naïve, memory, or effector subsets compared to control T cells from C57BL6 LRP1 ^fx/fx mice. However, after activation with anti-CD3/CD28, LRP1 KO T cells showed delayed entry into S-phase at 48 hours compared to C57BL6 LRP1 ^fx/fx T cells (p = 0.06), although there was no difference by 72 hours. This data supports a novel role for LRP1 in regulating activation-induced T cell proliferation and potentially explains how LRP1-mediated effects on T cell proliferation results in tolerance with incomplete donor chimerism and prevention of GVHD. This points to LRP1 as a potential target for regulating T cell function and preventing GVHD.

No relevant conflicts of interest to declare.