Characterization of the humoral immune response during Staphylococcus aureus bacteremia and global gene expression by Staphylococcus aureus in human blood

Abstract

Attempts to develop an efficient anti-staphylococcal vaccine in humans have so far been unsuccessful. Therefore, more knowledge of the antigens that are expressed by Staphylococcus aureus in human blood and induce an immune response in patients is required. In this study we further characterize the serial levels of IgG and IgA antibodies against 56 staphylococcal antigens in multiple serum samples of 21 patients with a S. aureus bacteremia, compare peak IgG levels between patients and 30 non-infected controls, and analyze the expression of 3626 genes by two genetically distinct isolates in human blood. The serum antibody levels were measured using a bead-based flow cytometry technique (xMAP®, Luminex corporation). Gene expression levels were analyzed using a microarray (BµG@s microarray). The initial levels and time taken to reach peak IgG and IgA antibody levels were heterogeneous in bacteremia patients. The antigen SA0688 was associated with the highest median initial-to-peak antibody fold-increase for IgG (5.05-fold) and the second highest increase for IgA (2.07-fold). Peak IgG levels against 27 antigens, including the antigen SA0688, were significantly elevated in bacteremia patients versus controls (P≤0.05). Expression of diverse genes, including SA0688, was ubiquitously high in both isolates at all time points during incubation in blood. However, only a limited number of genes were specifically up- or downregulated in both isolates when cultured in blood, compared to the start of incubation in blood or during incubation in BHI broth. In conclusion, most staphylococcal antigens tested in this study, including many known virulence factors, do not induce uniform increases in the antibody levels in bacteremia patients. In addition, the expression of these antigens by S. aureus is not significantly altered by incubation in human blood over time. One immunogenic and ubiquitously expressed antigen is the putative iron-regulated ABC transporter SA0688.

Figure 1. Dendogram of clinical isolates.

Pulsed-field gel electrophoresis data and spa-types of S. aureus isolates obtained from blood cultures of 21 bacteremia patients are shown.

Figure 2. Antibody courses against the antigen SA0688 in bacteremia patients.

A: Courses of anti-SA0688 IgG levels in 21 bacteremia patients. Each data point represents the average of duplicate Luminex measurements expressed as the mean fluorescence intensity (MFI). Each patient is represented by a serial set of the same symbols and a polynomial trend lines is drawn through this set. B: Course of anti-SA0688 IgA levels in 21 bacteremia patients. Note that the MFI values on the Y-axis are a factor of 10 lower than in Figure 2A.

Figure 3. Initial-to-peak fold-increases in IgG levels for 6 bacterial antigens in bacteremia patients.

Each data point represents a single patient; the median fold-increase in IgG levels and interquartile range are represented by lines. Patients for whom the duplicate Luminex measurements had a CV >25% were excluded from the analysis. Note the log10 scale of the y-axis.

Figure 4. Comparison of anti-SA0688 IgG levels in bacteremia patients and non-infected controls.

Peak IgG levels of 21 bacteremia patients were compared to IgG levels of 30 non-infected controls. The median value and interquartile range are represented by lines. Note the log10 scale of the y-axis.

Figure 5. Functional distribution of genes with altered mRNA expression in human blood.

The functional classes are shown for which the largest number of genes showed an at least twofold increased or decreased mRNA expression at all timepoints in blood (30, 60 and 90 minutes) in both strains compared to trancriptomes at the start of incubation in blood (0 minutes). Functional classes for which only one gene showed significant alterations in mRNA expression in blood are not shown.