Anti-IsdB Antibody-Secreting Cells Found in S. aureus-Infected Periprosthetic Bone Marrow: Case Series of Total Hip Arthroplasty Patients.

Staphylococcus aureus periprosthetic joint infection is notoriously difficult to treat and is often associated with septic death. Results from the V710 Phase IIB/III clinical trial and subsequent research have demonstrated that non-neutralizing antibodies against the iron-regulated surface determinant protein B (IsdB) facilitate bacterial entry into leukocytes, generating "Trojan horse" leukocytes and the dissemination of surgical-site infections. In contrast, anti-glucosaminidase (Gmd) antibodies, which mediate opsonophagocytosis of bacterial clusters, are associated with protection from S. aureus osteomyelitis in both mice and humans. To further test whether anti-IsdB antibodies are pathogenic and anti-Gmd antibodies are protective in infected patients, we performed a clinical pilot study of three healthy patients undergoing primary total hip arthroplasty and a patient undergoing revision arthroplasty for S. aureus infection to assess the feasibility of quantifying anti-IsdB and anti-Gmd antibody-secreting cells in blood and periprosthetic bone marrow. We also assessed anti-IsdB and anti-Gmd antibody levels in serum, blood plasma, bone marrow plasma, and cell culture supernatant collected after six days with and without a previously established memory B-cell stimulation cocktail. The frequencies of memory B-cells and antibody-secreting plasmablasts were measured using IgG ELISpot. Consistent with our hypothesis, we found large numbers of anti-IsdB antibody-secreting cells in unstimulated periprosthetic bone marrow from the infected patient, which increased with stimulation. However, these cells were only detectable in peripheral blood mononuclear cells following stimulation. Furthermore, anti-Gmd antibody-secreting cells were undetectable in the bone marrow of this infected patient at the time of revision surgery. This patient also had very high titers of anti-IsdB antibodies in serum, blood plasma, and bone marrow plasma. Anti-IsdB and anti-Gmd antibody-secreting cells were only detected in bone marrow for one of the uninfected patients. Collectively, the results are consistent with the hypothesis that anti-IsdB antibodies are associated with disease and related to the presence of IsdB-specific memory B-cells and plasmablasts. This case series demonstrates the utility of ELISpot to assess local vs. systemic antigen-specific humoral responses in total hip arthroplasty patients and support a larger study to test the hypothesis that anti-IsdB antibody-secreting cells proximal to S. aureus infected implants are associated with recalcitrant infections and septic death.