Recombinant Human Mannose-Binding Lectin (rhMBL) Binds to Human and Mouse Cancer Cells and Elicits Antitumor Activity in a MBL Knockout Mouse Model of Murine Cancer.

Mannose-binding lectin (MBL), a human plasma protein, plays a role in innate immune defense. MBL recognises microorganisms through surface carbohydrate structures leading to either direct opsonisation or activation of complement system. Since MBL deficiency is associated with increased susceptibility to infections in immunosuppressed individuals, e.g., during chemotherapy induced neutropenia, replacement therapy with recombinant human MBL (rhMBL) may be beneficial in this patient group. MBL has also been shown to bind to cancer cell lines. Therefore, we hypothesized that MBL binding to cancer cells may lead to direct opsonisation or activation of the innate immune system leading to cell kill. In this study, we aim to determine the binding and therapeutic efficacy of rhMBL in cancer cells and animals models respectively. The in vitro binding of rhMBL was evaluated on human cancer cell lines by flow cytometry using a specific anti-MBL antibody. rhMBL demonstrated binding to 6 of 7 cancer cells lines tested. These included human ovarian (OVCAR-3, SK-OV-3), pancreatic (MiaPaCa-2), colorectal (COLO-205), lymphoma (Daudi) and murine melanoma (B16-F0) cancer cells. The binding of rhMBL appeared to be specific since it was partially inhibited by 100mM mannose, 100mM N-acetyl glucosamine, 20mM mannan, use of Ca2⁺-free buffer or with 10mM EDTA. The antitumor effects of rhMBL were evaluated in MBL knockout (MBL/KO) transgenic mice, which lack both MBL-1 and MBL-2 genes, and corresponding MBL wildtype mice injected subcutaneously with B16-F0 mouse melanoma cells. MBL wildtype mice injected with B16-F0 mouse melanoma cells did not develop subcutaneous tumors. However, MBL/KO mice had robust tumor growth suggesting that MBL deficient individuals may be more susceptible to development of cancers.

Treatment of MBL/KO mice with 75 μg MBL/mouse (q2dx5) resulted in 63% tumor growth inhibition compared with the control mice (P<0.05). Moreover, MBL/KO mice treated with rhMBL had a significant prolongation of life span compared with the control group (P<0.05). In conclusion, rhMBL binds to mouse and human tumor cells and inhibits tumor cell growth in a MBL/KO murine melanoma model. This phenomenon may not take place in MBL-deficient individuals, and these individuals may thus be more susceptible to development of tumors. Replacement therapy with rhMBL in these individuals may have anticancer effects. rhMBL should thus be further evaluated for prevention and treatment of infection complications in MBL deficient immunocompromised patients as well as potential anticancer therapy.