Rapid T Cell Response to Vaccination Can Occur without Antibody Response in Children Post HSCT.

Vaccination is widely used to improve pathogen-specific immunity in patients post HSCT, but it is not known whether patients can mount an effective T cell response to vaccine antigens (vAg). Moreover the relationship between T and B cell response to vAg has not been studied. We hypothesized that a sufficiently sensitive assay of T cell response to vAg would allow vaccination to be used as a tool to measure immune recovery post HSCT and improve vaccine design. We therefore: (1) developed a flow-cytometry-based approach to quantify and characterize T cells specific for vAg; (2) validated it by measuring T cell immunity to influenza A in normal donors; and (3) characterized the T and B cell response to influenza vaccination in pediatric HSCT patients. PBMC were labeled with CFSE and stimulated in vitro with whole influenza Ag. Ag-specific T cells were sensitively detected by their proliferation (loss of CFSE fluorescence) and simultaneous expression of the activation marker HLA-DR. Proliferating/active T cells could be readily detected after stimulation with influenza A Ag in healthy adult (n=4) and pediatric (n=19) donors but were absent in control conditions. Both CD4+ and CD8+ T cell proliferation was detected in all donors but one, and in children as young as 6mo. Staining with MHC I- and MHC II-tetramers confirmed that the proliferating/active population contained T cells specific for immunodominant CD8+ and CD4+ epitopes, demonstrating that vAg were processed and presented to epitope-specific T cells. To characterize the phenotype of influenza-specific T cell memory, we separated memory and naive CD4+ cells prior to antigen-stimulation. Antigen-experienced (CD45RA−/CCR7−) but not naive (CD45RA+/CCR7+) T cells proliferated to vAg confirming that the assay detected pre-existing influenza-A-specific T cell memory. We next assessed Influenza-A-specific T cell immunity before and after influenza vaccination in five pediatric HSCT recipients (mean age 10.6y, range 5–15y; mean time from transplant 13m, range 3–21m). Prior to vaccination the CD4 proliferation to influenza-A was a mean of 3.3% (range 0.04–11%). Following vaccination CD4 proliferation increased significantly in all patients (mean 19.0%, range 6.9%–31.8%, p=0.02). This increase was specific as proliferation to control Ag was unchanged. Influenza-A CD8+ proliferation also increased in 3 of 5 patients but was not statistically significant for the group consistent with the limited efficacy of soluble vAg in inducing CD8+ T cell response. All patients had detectable influenza-A-specific IgG levels prior to vaccination but despite a T cell response to vaccination in all patients, none had a significant increase in IgG level following vaccination. Only one patient had an IgM response; this patient also had the highest influenza-A-specific CD4 proliferation before and after immunization suggesting that there may be a threshold of T cell response required for a B cell response. Using a novel assay we demonstrate that a T cell response to vaccination can occur without an accompanying B cell response. This assay provides a more sensitive measure of immunity to vaccination and allows vaccine response to be used as a benchmark of strategies to accelerate post-HSCT T cell reconstitution.