Inhibitory Ligands CD200, CD270, CD274 and CD276 Are Expressed On Eμ-TCL1 Transgenic Mouse Splenocytes and Are of Potential Relevance to Impaired T-Cell Function in Vivo
Authors: McClanahan Fabienne , Ghirelli Cristina , Greaves Paul , Riches John C. , Coutinho Rita , Ramsay Alan G. , Gribben John G 
Published: October 2019

Abstract
Abstract 313

Background:
We have previously demonstrated that CD4 and CD8 T-cells from CLL patients show profound dysfunctions in multiple gene pathways, including the actin cytoskeleton, which impairs the formation of functional immunologic synapses between T cells and APCs. Functional screening assays on Mec-1 cells have identified CD200, CD270, CD274, and CD276 as inhibitory ligands which induce impaired actin synapse formation in both allogeneic and autologous T cells. We also demonstrated that the Eμ-TCL1 transgenic mouse model of CLL closely resembles the T-cell defects observed in humans, validating it as a valuable preclinical tool to examine changes in the microenvironment alongside the development of leukaemia. The aim of the current study is to investigate the role of CD200, CD270, CD274, and CD276 in the Eμ-TCL1 model.

Methods:
We used multiparameter flow cytometry to establish the expression of inhibitory ligands on CD19+/CD5+ unpurified splenocytes from Eμ-TCL1 mice on both the C57Bl/6 (B6) and the C3HB6-F1 background and compared this to unpurified splenocytes from age matched wild-type (WT) controls of the respective coisogenic strain.

Results:
A total of 19 leukemic Eμ-TCL1 (n=10 C57Bl/6 and n=9 C3HB6-F1 background) and 11 WT mice (n=6 C57Bl/6 and n=5 C3HB6-F1 background) were examined. CD19+/CD5+ CLL cells constituted 92% (range 62–97%) of the DAPI-negative lymphocyte population. On CD19+/CD5+ CLL cells, CD274 (mean 98% ± SEM 0.4) and CD200 (mean 84% ± SEM 2.9 were uniformly strongly expressed, while CD270 (mean 74% ± SEM 4.7) and CD276 (mean 50% ± SEM 6.6) showed a weaker and more diverse expression, with no significant differences between the two backgrounds (all p>.05). Similar expression patterns were observed in Eμ-TCL1 mice with spontaneously occurring CLL and transplanted transgenic mice, with no differences between spontaneous and induced CLL (all p>.05). We then compared transgenic CD19+/CD5+ CLL cells to the WT CD19+ and the WT CD19+/CD5+ B1a-like cell population. Eμ-TCL1 CLL splenocytes showed a significant higher expression of CD274 and CD276 compared to expression on WT CD19+ (p<.0001, p=.00349) splenocytes. When compared to WT B1a-like splenocytes, only CD274 was significantly higher expressed (p<.0001). To clarify the impact of genetic strain, B6 and C3HB6-F1 were investigated separately: transgenic mice on the B6 background showed significantly higher expression of CD274 compared to WT B6 CD19+ (p=.0015) and WT B6 B1a-like (p<.0001) splenocytes. In contrast, transgenic mice on the C3HB6-F1 background showed a significant higher expression of CD274 and CD276 compared to WT CD19+ (p=.0002, p=.00354) and WT B1a-like (p=.0005, p=.00384) splenocytes. These patterns substantiate differences of the expression of inhibitory ligands between the WT strains, but of note, these were not mirrored in TCL1 mice.
In previous experiments, we used the Eμ-TCL1 model to investigate the polarization of F-actin and phosphotyrosine at the immune synapse between splenic autologous T-cells and APCs and subsequent effector function. Age-matched WT mice had a significantly higher accumulation than transgenic mice. To assess the functional role of inhibitory ligands, knock-down experiments using lentiviral shRNA and blocking antibodies are currently under way to assess if this restores immune synapse formation and T cell effector function in vivo.

Conclusions:
The inhibitory ligands CD200, CD270, CD274 and CD276 are expressed in vivo and appear to be of functional relevance for the anti-cancer immune response. They therefore represent attractive targets to restore T-cell effector function, which might be achieved by gene therapy approaches and blocking antibodies.

Disclosures:
Gribben: Celgene: Honoraria.