|Screen||T-dependent Antibody Response to rSFV-bgal Screen|
|Posted On||02/18/2010 12:24 PM|
|Author||Jin Huk Choi, Samantha Kuwada Teixeira|
|Science Writer||Eva Marie Y. Moresco|
This screen is designed to identify genes required for the in vivo B cell response to a model antigen encoded by a recombinant Semliki Forest Virus vector. Mice are immunized on day 0, boosted on day 14, and serum is collected and analyzed for antigen-specific IgG on day 28. ENU-mutatgenized G3 mice that produce a reduced amount of IgG to a strong antigen or a detectable amount of IgG to a weak antigen (see below) relative to wild type mice are identified as potential mutants.
During a T cell-dependent humoral immune response, a subset of activated CD4+ cells -- follicular homing T cells or T-FH -- migrate to the T-B borders of secondary lymphoid organs, and interact with cognate antigen-specific conventional (B-2) B cells that have been activated by soluble antigen, and stimulate them through CD40-CD40 ligand (1) and other interactions to expand within T cell or B cell zones. B cells in the T cell zone develop into short-lived plasma cells that produce germline-encoded antigen-specific antibody. In contrast, B cells expanding in B cell zones generate regions called secondary follicles, precursors of the germinal center (GC) reaction. As the GC reaction progresses, it becomes polarized into separate “dark” and “light” zones. B cells proliferate and diversify via somatic hypermutation within the dark zone, and then migrate to the light zone, where B cells with high-affinity variant Ig receptors are selected to either re-enter the GC cycle or exit the GC reaction. Cells exit the GC reaction as either long-lived plasma cells or recirculating memory B cells. Long-lived plasma cells produce large amounts class-switched, high-affinity antibody. Memory B cells do not actively secrete antibody. Instead, these cells are programmed for expansion and differentiation into high-affinity plasma cells upon secondary encounter with antigen, the hallmark of the memory B cell response.
Single-round infectious recombinant viral vectors, including recombinant Semliki Forest Virus (rSFV) vaccine vectors (2-5), induce both cellular and humoral immune responses. SFV is an alphavirus genetically related to Sindbis virus and Venezuelan equine encephalitis virus, for which single-round replicon systems have also been developed (6-8). The vector used here contains the SFV translation-enhancer element upstream and in frame with an internal signal sequence and the target antigen-encoding sequence (9). This vector was shown to result in secretion of ten-fold more antigen compared to the same vector lacking the enhancer element and using the N-terminal CD5 signal sequence, as well as improved antibody titres in the sera of immunized mice (9). In the protocol outlined here, the rSFV vector encodes either the model antigen β-galactosidase (GAL) or ovalbumin (OVA) and is administered via intraperitoneal injection. Detection of GAL- or OVA-specific IgG two weeks after administration of a second dose of the vector is used as a readout for the class-swiched B cell response. Most wild type mice mount a robust GAL-specific IgG response, and mutagenized mice that fail to do so may possess mutations in genes required for B or CD4+ T cell development or activation, isotype class switching, or terminal differentiation. By contrast, wild type mice fail to produce detectable OVA-specific IgG responses; of the strains tested, only interferon (IFN)α receptor-deficient mice mount an OVA-specific IgG response. Mutagenized mice that are able to respond may have mutations in the type I IFN signaling pathway or other genes that normally downregulate weak B cell responses.
|Reagents and Solutions|
β-galactosidase (Roche Applied Science, Indianapolis, IN)
ovalbumin (Sigma-Aldrich, St. Louis, MO)
HRP-conjugated goat-anti-mouse IgG (Southern Biotech, Birmingham, AL)
TMB SureBlue reagent (KPL, Inc., Gaithersburg, MD)
TMB Stop Solution (KPL, Inc., Gaithersburg, MD)
1. On day 0, dilute 2x106 infectious units (IU) of rSFV vector encoding β-galactosidase (GAL) in a total volume of 200 μl 0.9% saline and inject intraperitoneally into each G3 mouse.
Measurement of antigen-specific IgG by ELISA
2. Prepare an ELISA plate. Coat a 96-well plate overnight at 4°C with 2 μg/mL β-galactosidase in PBS.
3. On day 14 after immunization, collect about 50 μl of blood from the sub-mandibular vein of each mouse.
4. Perform ELISA according to standard protocol:
a) Snap plate to remove the coating antigen.
b) Wash plate 4 times with 200 μL of 0.05% PBST per well.
c) Add 200 μL of 1% (w/v) BSA in PBS per well.
d) Wrap or cover plate and incubate at least 30 minutes at 37°C.
e) Snap plate to remove the blocking buffer.
f) Add 150 μL of 1% (w/v) BSA in PBS per well to the wells of rows A, C, E, G.
g) Add 100 μL of 1% (w/v) BSA in PBS per well to the wells of rows B, D, F, H.
h) Add 3ul of sera to wells containing 150 ul of 1% (w/v) BSA in PBS (1:50 dilution) and mix well, then transfer 50 ul of volume from rows A, C, E, G to B, D, F, H, respectively, to make 1:150 dilution. Mix well and discard the last 50 ul of the volume (prepare serum dilutions in 96 well plates).
i) Wrap or cover plate and incubate for 2 hours at 37°C.
j) Wash plate 8 times with 0.05% PBST.
k) Add 100 μL of HRP-conjugated goat-anti-mouse IgG diluted 1:4,000 in PBS per well.
l) Wrap or cover plate and incubate for 1 hour at 37°C.
m) Wash plate 8 times with 0.05% PBST.
n) After last wash has been removed, develop plate by adding 100 μL of room temperature TMB SureBlue reagent per well.
o) Incubate for 1 to 5 minutes at room temperature.
p) Stop the reaction by adding 100 μL of TMB Stop Solution per well.
q) Read absorbance at 450 nm.
|Critical Parameters and Troubleshooting|
Sera from immunized mice are pooled and aliquoted to make a “positive control” stock of sera. The positive control stock is diluted in the same way as experimental samples, and run on a column of uncoated wells and on a column of coated wells on every plate. Running the positive control sera on mock-coated wells (i.e., wells coated with PBS alone) establishes the background for the plate. Running the positive control sera on coated wells establishes the plate-to-plate variation for the assay.
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