Yellow Fever Vaccine

Yellow Fever (YF) vaccine is a live attenuated vaccine, meaning that genetic modifications to the virus result in little or no disease symptoms while conferring protective immunity. Indeed, YF vaccine is considered to be one of the most effective vaccines in use. The graph below illustrates the ability of the MIMIC® System to support the induction of CD4 helper cell activity to YF vaccine in naïve individuals. In this instance we observed ten (10) donors whose APC and CD4 cells were cultured in the presence or absence of YF vaccine. Cells were subsequently challenged with APC expressing YF antigen and enumerated with respect to simultaneous expression of interferon gamma (IFNg) a marker of T cell activation and CD154 (aka CD40 ligand) a correlate of antigen specific activation. As anticipated, the MIMIC® System cultures primed and challenged with YF vaccine demonstrated a significant increase in YF specific CD4 T cell responsiveness, though some variability of response was noted between donors.

Chart - MIMIC® System cultures primed and challenged with YF vaccine demonstrated a significant increase in YF specific CD4 T cell responsiveness, though some variability of response was noted between donors.

As part of the YF study we evaluated the kinetics of the CD4 response in an ex vivo system. Here, six (6) subjects were monitored for CD4 proliferation, a surrogate marker of activation, for thirty days post immunization with YF vaccine. It was observed that CD4 proliferation increased to its peak activity about fourteen (14) days post-vaccination. At this time, there was a significant increase in the percentage of CD4 T cells proliferating as compared with pre- vaccination levels. Note that Ki67 is a cellular marker for proliferation. Sekaly et al., JEM, Vol. 205, No. 13, 3119-3131 (2008)

Chart - CD4 proliferation increased to its peak activity about fourteen (14) days post-vaccination

A hallmark of CD4 T cell activation is the stimulation and secretion of cytokines. Cytokines substances that are secreted by specific cells of the immune system to carry signals locally between cells, and thus have an effect on other cells. It has been shown that CD4 cells may be biased in the species of cytokines they secrete. For example, CD4 T cells that express IFNg and IL2 (interleukin two) are termed Th1 cells and are particularly effective against intracellular pathogens. In contrast, CD4 cells expressing IL4, IL5 and/or IL13 are defined as Th2 cells which play a role against extracellular pathogens such as parasites. In the current study, we examined the capacity of YF vaccine to induce a range of cytokines in the MIMIC® System. Here, we observed that YF vaccine elicited a robust cytokine response inducing a mixed Th1 and Th2 response. These findings are supportive of the ability of YF vaccine to generate both cellular and humoral protective immunity and are in good agreement with what is known about the in vivo effects of the vaccine.

Observations in the MIMIC® System on patterns of cytokine production were corroborated in ex vivo studies of twenty (20) volunteers administered YF vaccines who were probed for patterns of cytokine expression using YF peptide pools. Here, ex vivo responses illustrated: (1) a mixed Th1 and Th2 response profile stimulated by YF peptides; and (2) the donor to donor variability of response profile underscoring the need to observe human responses in a number of individuals. The ex vivo data is in good agreement with that observed in the MIMIC® System. Sekaly et al., JEM, Vol. 205, No. 13, 3119-3131 (2008); Santos, et. al., Cytokine, 2008, vol. 42. pp. 152

In addition to CD4 responses, we also examined the ability of YF vaccine to elicit a CD8 cytotoxic T cell response. While CD4 helper cells orchestrate much of the adaptive immune response, CD8 cytotoxic cells are imbued with the ability to kill infected cells and play a significant role in the clearance of infection. In this study, we assessed the capacity of the MIMIC® System to support naïve induction of YF-specific CD8 cells. The figure below depicts a flow cytometric analysis of the CD8 response of three (3) donors to YF vaccine stimulation. In this study, cells were cultured in the presence or absence of YF and then probed for simultaneous formation of IFNg and CD107a (a surrogate marker for degranulation of CD8 cells with resultant cell killing). The percentage of cells in the upper right hand quadrant of the dot plots indicates that in donors 1 and 3, a notable induction of CD8 activity was manifest. Donor 2, while responsive, presented with a relatively high degree of background response.

CD8 cells were also evaluated in an ex vivo format for the kinetics of proliferation. CD8 activation/proliferation occurred rapidly following immunization, peaking on about day 14. When measured at the day 14 peak, a significant number of CD8 cells were seen to proliferate when compared with pre-vaccination frequencies. The ex vivo datasets are in good agreement with that found in the MIMIC® System; the YF vaccine elicits CD8 T cell responses. Sekaly et al., JEM, Vol. 205, 3119-3131 (2008); Reinhardt, J Med Virol. Vol. 56, 159-67 (1998)

Two cohorts of YF vaccines were assessed genomically for transcription factors resultant of YF immunization. Ex vivo responses of these two cohorts from Montreal, Canada and Lausanne, Switzerland were compared with profiles observed for in vitro MIMIC® cultures. While there was a degree of variability noted between the two in vivo YF cohorts and between in vivo and in vitro immunization schema, there was a uniform elicitation of several master switch genes (i.e., IRF1, IRF7, IRF8, STAT and FOXO3a) which are required for coordination of an effective response. Thus in vivo immunization of two geographically disparate groups and in vitro immunization in the MIMIC® System demonstrated a degree of variability in genomic activation but shared a number of critical transcription factors for mounting a protective response. Sekaly et al., Journal of Experimental Medicine, Vol. 205, No. 13, 3119-3131 (2008)

Whole Blood Montreal VaxDesign Whole Blood Lausane
BACH1
BACH2
CBFA2T2
CEBPA
CEBPB CEBPB
CRX
CUTL1
E2F1
E2F4
ETS2 ETS2 ETS2
FOXF2
FOXO1A
FOXO3 FOXO3
GATA1 GATA1
GATA4
GATA6
HNF4A
HSF1
IRF1 IRF1 IRF1
IRF2 IRF2
IRF7 IRF7 IRF7
IRF8 IRF8 IRF8
JUN JUN
LMO2
MLLT7 MLLT7
POU2F1
POU6F1
REL
RORA
SRF
STAT1 STAT1 STAT1
TAL1
TBP
TFAP4
TFDP1
VDR
VSX1