The Contribution of the membrane-bound complement regulatory proteins CD46 and CD55 in phases of acute lymphocytic leukemia (ALL) and acute myelogenous leukemia (AML)

Abstract

The complement system is an essential part of the innate immunity where it is involved in the elimination of pathogens and disposal of apoptotic bodies and immune complexes from the body. Membrane-bound complement regulatory proteins (mCRPs) play key roles in controlling complement activity to avoid any accidental damage of host cells. The role of the complement system during the neoplastic transformation of cells is complicated and has been debated for long. On one hand, the complement system generally acts as a participant in the bodys immune surveillance against cancer. However, recent findings have shown that cancerous cells can use the complement components to assist in certain hallmarks which are fundamental for tumor progression such as angiogenesis, proliferation and metastasis. The aim of the current study is to investigate the differential expression of mCRPs; CD46 and CD55 in of acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML). Clinical peripheral blood samples of newly diagnosed AML and ALL patients were used to assess the changes in transcriptional expression levels of CD46 and CD55 compared to age-matched healthy control subjects using quantitative real time PCR (qPCR). Results showed that both CD46 and CD55 were significantly downregulated by 2 to 7 folds in ALL and AML patients compared to healthy controls. The determined downregulation is suggestive of a defense mechanism conducted by leukemic cells to overcome immune defenses. Flow cytometric analysis was conducted for proteomic expression analysis of both proteins on cell surfaces of leukemia patients compared to healthy controls and results showed a reduction in CD46 expression by 1.2 fold and 2.8 fold in CD55 expression in AML patients. Post transcriptional knockdown of both genes was then carried out in HSB-2 leukemic cell model using customized shRNA, followed by flow cytometry analysis to assess the success of CD46 and CD55 silencing step, and ending with cell viability assays. MTT assay results showed a significant reduction in the viability of HSB-2 cells by 3 fold, approximately, following post-transcriptional silencing of CD46 and CD55 suggesting that although the expression of these mCRPs could be normally compromised by cancerous cells to evade complement attack mechanisms, CD46 and CD55 could be vital to the viability and proliferation of cancerous cells at some point in time. Our results suggest the dual role of complement in the tumor microenvironment where a balance between antitumor and tumor-promoting complement activities exists. The multifunctional properties of the complement system could be employed in opposing roles in cancer suggesting that the biological functions of the complement system are much more diverse than a simple elimination of target cells.