Phagocytosis via complement or Fc-gamma receptors is compromised in monocytes from type 2 diabetes patients with chronic hyperglycemia

Abstract

Type 2 diabetes patients (DM2) have a higher risk of tuberculosis (TB) that may be attributed to functional defects in their mononuclear phagocytes given the critical role of these cells in Mycobacterium tuberculosis containment. Our previous findings suggest that monocytes from DM2 have reduced association with serum-opsonized M. tuberculosis. To determine if this alteration is due to defects in phagocytosis via complement or Fc-gamma receptors (FcγRs), in this study we evaluated the uptake of sheep red blood cells coated with IgG or complement, respectively, by monocytes from individuals with and without DM2. We found that chronic hyperglycemia was significantly associated with reduced phagocytosis via either receptor by univariable and multivariable analyses. This defect was independent of host serum opsonins and flow cytometry data indicated this was not attributed to reduced expression of these phagocytic receptors on DM2 monocytes. The positive correlation between both pathways (R = 0.64; p = 0.003) indicate that monocytes from individuals with chronic hyperglycemia have a defect in the two predominant phagocytic pathways of these cells. Given that phagocytosis is linked to activation of effector mechanisms for bacterial killing, it is likely that this defect is one factor contributing to the higher susceptibility of DM2 patients to pathogens like M. tuberculosis.

Figure 1. Correlations between FcγR- and CR-mediated phagocytosis and host characteristics.

sRBCs were coated with C3b/iC3b or IgG and the percentage of monocytes with at least one phagocytosed sRBC was established by fluorescence microscopy among participants with and without DM2. The correlation between phagocytosis via CRs for C3b/iC3b-coated sRBCs and via FcγRs for IgG-coated sRBCs is shown with respect to HbA_1c, or two other host characteristics that are also distinct in DM2 patients: age and BMI. Correlation coefficients (R) and corresponding p values are provided, and the linear regression displayed. Black circles, no DM2; gray circles, DM2.

Figure 2. Correlation between CR- and FcγR-mediated phagocytosis.

Phagocytosis of sRBCs coated with C3b/iC3b or IgG is described in the Methods and the linear relationship between both phagocytosis pathways is displayed. Correlation coefficients (R) and corresponding p value is provided. Black circles, no DM2; gray circles, DM2.

Figure 3. Relationship between host factors and the median fluorescence intensity (MFI) of phagocytosis-promoting and -inhibiting receptors.

The mean of the MFI for monocyte receptors was established for all the CD14-positive monocytes or the classical, intermediate and non-classical monocytes as described in the Methods and shown in Table S2. Three host factors were associated with changes by univariable and multivariable analysis among all monocytes (gender, BMI and DM2 in columns), and their relationship with CD32, CD64 and CD32B is illustrated (rows): male gender with decreased CD32 but not CD64 or CD32B, higher BMI with increased CD64 but not CD32 or CD32B, and DM2 with increased CD32B expression. Correlation coefficients (R) and corresponding p values are provided, and the linear relationship displayed for BMI. Horizontal line shows mean by DM2 status.