Cell-free IgG-aggregates in plasma of patients with chronic lymphocytic leukemia cause chronic activation of the classical complement pathway

Increased serum level of alpha-2 macroglobulin and its production by B-lymphocytes in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL), the most common adult’s leukemia in the western world, is caused in 95% of the cases by uncontrolled proliferation of monoclonal B-lymphocytes. The complement system in CLL is chronically activated at a low level via the classical pathway (CP). This chronic activation is induced by IgG-hexamers, which are formed after binding to alpha-2-macroglobulin (A2M). The study investigated for the first time the serum levels of A2M in CLL patients, their association with the disease severity, and A2M production by the malignant B-lymphocytes. Blood samples were collected from 65 CLL patients and 30 normal controls (NC) subjects, and used for quantifications of the A2M levels, the complement activation marker (sC5b-9), the complement components C2, C3 and C4, and clinical biochemistry and hematology parameters. The production of A2M was studied in B-lymphocytes isolated from blood samples as well as in CLL and non-CLL cell lines.The serum A2M levels were significantly higher in CLL patients vs NCs, showing values of 3.62 ± 0.22 and 1.97 ± 0.10 mg/ml, respectively. Within the CLL group, A2M levels correlated significantly with the disease stage, with sC5b-9, and with clinical indicators of the disease severity. Increased A2M production was showed in three out of four CLL B-lymphocytic lines that were studied, as compared to non-CLL lines, to a non-lymphocytic line, and to blood-derived primary B-lymphocytes. A2M production was further increased both in primary cells and in the CLL cell-line after incubation with CLL sera, compared to NC sera. This study shows for the first time that serum A2M levels in CLL are significantly increased, likely due to A2M production by the malignant B-lymphocytes, and are correlated with the disease severity and with chronic complement activation. The moderate change in A2M production after incubation with NC sera in-vitro supports the hypothesis that inhibition of excess A2M production can be achieved, and that this may potentially down-regulate the IgG-hexamerization and the resulting chronic CP activation. This may also help restore complement system activity, and eventually improve complement activity and immunotherapy outcomes in CLL.

The Role of Alpha 2 Macroglobulin in IgG-Aggregation and Chronic Activation of the Complement System in Patients With Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is the most common leukemia in adults in the western world. One of the treatments offered for CLL is immunotherapy. These treatments activate various cellular and biochemical mechanisms, using the complement system. Recently it was shown that the complement system in CLL patients is persistently activated at a low level through the classical pathway (CP). The mechanism of chronic CP activation involves the formation of IgG-hexamers (IgG-aggregates). According to recent studies, formation of ordered IgG-hexamers occurs on cell surfaces via specific interactions between Fc regions of the IgG monomers, which occur after antigen binding. The present study investigated the formation of IgG-hexamers in CLL patients and normal (non-malignant) controls (NC), their ability to activate complement, their incidence as cell-free and cell-bound forms and the identity of the antigen causing their formation. Sera from 30 patients and 12 NC were used for separation of IgG- aggregates. The obtained IgG- aggregates were measured and used for assessment of CP activation. For evaluation of the presence of IgG- aggregates on blood cells, whole blood samples were stained and assessed by flow cytometry. Serum levels of IgG- aggregates were higher in CLL and they activated the complement system to a higher extent than in NC. Alpha 2 macroglobulin (A2M) was identified as the antigen causing the hexamerization/aggregation of IgG, and was found to be part of the hexamer structure by mass spectrometry, Western blot and flow cytometry analysis. The presence of A2M-IgG-hexamers on B-cells suggests that it may be formed on B cells surface and then be detached to become cell-free. Alternatively, it may form in the plasma and then attach to the cell surface. The exact time course of A2M-IgG-hexamers formation in CLL should be further studied. The results in this study may be useful for improvement of current immunotherapy regimens.