Salmonella Infection Drives Promiscuous B Cell Activation Followed by Extrafollicular Affinity Maturation

Abstract

The B cell response to Salmonella typhimurium (STm) occurs massively at extrafollicular sites, without notable germinal centers (GCs). Little is known in terms of its specificity. To expand the knowledge of antigen targets, we screened plasmablast (PB)-derived monoclonal antibodies (mAbs) for Salmonella specificity, using ELISA, flow cytometry, and antigen microarray. Only a small fraction (0.5%-2%) of the response appeared to be Salmonella-specific. Yet, infection of mice with limited B cell receptor (BCR) repertoires impaired the response, suggesting that BCR specificity was important. We showed, using laser microdissection, that somatic hypermutation (SHM) occurred efficiently at extrafollicular sites leading to affinity maturation that in turn led to detectable STm Ag-binding. These results suggest a revised vision of how clonal selection and affinity maturation operate in response to Salmonella. Clonal selection initially is promiscuous, activating cells with virtually undetectable affinity, yet SHM and selection occur during the extrafollicular response yielding higher affinity, detectable antibodies.

Figure 1. STm Infection Induces a Rapid, Massive AFC Response without GCs

(A) Spleen weight (red line) and splenic bacterial burden (black line) in infected mice over time. (B–E) Immunofluorescence staining of cry-osections from spleens of infected mice for GCs (B, day 11; C, day 28), AFC patches (D, day 4, arrows) and isotype-switched IgG2c AFCs (E, day 11). (F and G) Analysis of the abundance of AFCs by ELISpot (F) or of plasmablasts and GC cells by flow cytometry (G, gating as reported in Figure S1). Data shown here are representative of at least six independent experiments. In (A), (F), and (G), data are represented as mean ± SD of groups of at least five mice. See also Figure S1.

Figure 2. Screening of the Specificity of the AFC Response

(A and B) ELISpot assays for Ag-specific IgG AFCs (Ags: LPS, OmpA, STm lysate) shown as total number of cells per spleen (A) or as percentage of the IgG and IgM AFC compartment (B). (C) Schematic panel illustrating the hybridoma production and screening strategy and methodologies. In the example showed here, an IgG mAb which was selected in the first screening is then probed on the Ag microarray, where LppA and LppB (proteins with high homology) were identified as the antigenic targets. (D and E)Summary of the frequency of Ag-specific IgG (D) or IgM (E) mAbs from hybridomas identified after the screening. Data shown in (A) and (B) are representative of at least six independent experiments and are represented as mean ± SD of groups of at least five mice. See also Figure S2.

Figure 3. Infection of Genetically Modified Mouse Strains

(A–D) Assessment of the TLR-dependent component of the response. Wild-type B6, Myd88^−/− B6, and TLR2-TLR4^−/− B6 mice were infected and the AFC response was evaluated by Elispot for total IgG (solid lines in A) and IgM (dotted lines in A), LPS (B), STm lysate (C), and OmpA (D). Because infection with the standard dose used throughout our study resulted in high mortality rate in Myd88-deficient mice (not shown), these were infected with a lower amount of STm (2 × 10⁴), which permitted 100% survival after 30 days. (E–G) Mice with engineered Ab genes/BCR were infected and the splenic AFC response was measured for: total IgG in wild-type C57BL/6, B1.8^+/+(C57BL/6), or B1.8^+/+ Jk^−/− mice (C57BL/6) (E); total IgG (F) or OmpA-specific IgG (G) in wild-type Balb/c, Vk8R^+/+ (BALB/c), or Jk^−/− (Balb/c) mice. In all panels data are representative of at least two independent experiments and are represented as mean ± SD of groups of at least four mice.

Figure 4. Repertoire Characteristics of the Plasmablast Response to STm Infection

IgM sequences of naive follicular (FO) and marginal zone (MZ) B cells from uninfected mice were compared to IgM, IgG, and IgA sequences from plasmablasts 7 days (D7PB) and 21 days (D21PB) post-infection. (A) V family usage (x axis) shown for different isotypes as the percentage (y axis) of processed sequences within each sample. (B) Comparison of the Hill diversity index (^qD, y axis) over varying diversity orders q (see Supplemental Information), between IgM sequences from naive samples and IgM, IgG, and IgA sequences from samples of infected mice. The median diversity score over all resampling realizations is plotted as a line with the middle 95% percentile of the resampled distribution indicated by a shaded background. All samples were randomly downsampled to 800 sequences for each resampling realization to correct for variations in sequencing depth. (C) Comparison of Hill diversity indices between IgM sequences grown in cell culture under three conditions: untreated, cells that proliferated after CpG treatement (CpG div), and cells that did not proliferate after CpG treatment (CpG non-div). All samples were uniformly downsampled to 800 sequences for comparison to (C). (D–F) Violin plots showing the distribution of mutations from the germline (y axis) for each sample (x axis) and isotype. Only sequences with ≤ 10% mutation frequencies are shown. See also Figure S4 and Table S1.

Figure 5. Ongoing SHM Takes Place in Plasmablast Patches

(A) Example of a plasmablast patch pick by laser capture and microdissection (LCM); the same patch is shown before and after the pick. The black line indicates the area of ~20 cells that was dissected. The plasmablast patches are identified with anti-CD138 staining by immunohistochemistry. (B-D) Examples of three clonal trees of different complexity derived from the analysis of the Ab gene sequences obtained by LCM. The size of each node indicates the number of identical sequences found. In (D), a more complex tree is shown that was composed of sequences that derived from several nearby picks; different colors of the nodes denote different, but adjacent, picks from which the sequence was derived, while the number within the node indicates in how many serial slides (always from the same patch) the same sequence was found. The gray circle indicates an inferred intermediate. The position of the mutated nucleotides and aminoacids (in the case of replacement mutations) are shown along the branches. See also Figure S6 and Tables S2–S4.

Figure 6. Effect of SHM on Affinity in Cloned mAbs

(A) Number of mutations in Ab sequences from hybridoma clones. (B) Diagram indicating the relationships between the mAbs we isolated and the mutants we made, with respect to the heavy chain. The letters inside the circles represent amino acid substitutions as compared to the germline (GL, black circle on top). Circles on a gray background represent Abs that were found in the hybridoma screening; dotted circles on white background represent artificial intermediate versions. (C and D) Binding curves of recombinant mAbs expressed in their original or germline form, as well as in artificially mutated intermediate versions. The asterisks indicate the original sequences isolated from the hybridoma clones. An IgG sandwich ELISA was performed in parallel to confirm that all Abs were used in equal amounts in the assay (data not shown).