B cell maintenance of subcapsular sinus macrophages protects against a fatal viral infection independent of adaptive immunity

Abstract

Neutralizing antibodies have been thought to be required for protection against acutely cytopathic viruses, such as the neurotropic vesicular stomatitis virus (VSV). Utilizing mice that possess B cells but lack antibodies, we show here that survival upon subcutaneous (s.c.) VSV challenge was independent of neutralizing antibody production or cell-mediated adaptive immunity. However, B cells were absolutely required to provide lymphotoxin (LT) α1β2, which maintained a protective subcapsular sinus (SCS) macrophage phenotype within virus draining lymph nodes (LNs). Macrophages within the SCS of B cell-deficient LNs, or of mice that lack LTα1β2 selectively in B cells, displayed an aberrant phenotype, failed to replicate VSV, and therefore did not produce type I interferons, which were required to prevent fatal VSV invasion of intranodal nerves. Thus, although B cells are essential for survival during VSV infection, their contribution involves the provision of innate differentiation and maintenance signals to macrophages, rather than adaptive immune mechanisms.

Figure 1. Antibodies, but Not B Cells, Are Dispensable for Protection against Subcutaneous VSV Infection

(A) Kaplan-Meier survival curves of C57BL/6 (n = 9), BALB/c (n = 10), μMT (C57BL/6 background; n = 5), and D_HLMP2A (BALB/c background; n = 6) mice infected with 10⁶ pfu of VSV-IND i.v. D_HLMP2A versus μMT: p = 0.51, Log-rank (Mantel-Cox) test. (B) Kaplan-Meier survival curves of C57BL/6 (n = 93), BALB/c (n = 30), μMT (n = 50), and D_HLMP2A (n = 36) mice infected with 10⁴ pfu of VSV-IND i.fp. D_HLMP2A versus μMT: p < 0.0001, Log-rank (Mantel-Cox) test. (C) VSV neutralizing antibody titers in the sera of the mice described in (B), 15 days after infection. Graph is representative of two experiments (n = 4 per experiment); ND = not detected. (D and E) Ifnb1 (D) and Ifna2 (E) mRNA levels in total RNA isolated from LNs of the animals described in (B) and sacrificed 8 hr p.i. Analysis was performed by quantitative RT-PCR and results are expressed as relative units (RU) after normalization to the mRNA content of the housekeeping gene GAPDH. Graphs are representative of two experiments (n = 3 per experiment). **p < 0.001, *p < 0.05.

Figure 2. Surface Phenotype of SCS Macrophages in WT, μMT, and D_HLMP2A Mice

(A) Representative confocal micrographs of frozen popliteal LN sections stained with MAbs against SIGN-R1 (red), and CD169 (green). Scale bars represent 100 μm. (B) FACS histograms of SIGN-R1 expression on CD11b⁺CD169⁺ LN cells of the indicated mouse strains. Histograms are representative of three experiments (n = 3–4 mice/experiment). (C) Percentage of SIGN-R1⁺ cells within the CD11b⁺CD169⁺ compartment in LNs of the indicated mouse strains. ***p < 0.001.

Figure 3. B Cells Are Required for VSV Replication in LN Macrophages

(A) Representative confocal micrographs of popliteal LN sections 30 min after footpad infection with Alexa Fluor 488-labeled VSV (VSV488, green). Sections were stained with MAbs against CD169 (red) and B220 (blue). Scale bars represent 100 μm. (B) Viral titers in popliteal LNs of mice infected 30 min earlier with VSV-IND into the footpad. Results are representative of two experiments (n = 4 per experiment). (C) Representative confocal micrographs of popliteal LN sections 8 hr after footpad infection with VSV-eGFP (green). Sections were stained as in (A). Scale bars represent 150 μm. (D) Viral titers in popliteal LNs 8 hr after footpad infection with VSV-IND. Results are representative of three experiments (n = 3–4 mice/experiment).

Figure 4. LN Macrophage Depletion Renders Antibody-Deficient Mice Susceptible to Fatal VSV Neuroinvasion

(A) Ifnb1 (left), Ifna2 (middle), and Ifna4 (right) mRNA levels in total RNA isolated from LNs of the indicated mouse strains, with or without clodronate liposomes (CLL) treatment, 8 hr after footpad infection with VSV-IND. Analysis was performed as indicated in Figures 1D and 1E. Graphs are representative of two experiments (n = 3–4 per experiment). ***p < 0.001, *p < 0.05. (B) The same set of data as in (A) are expressed as: (mRNA levels in CLL-treated LNs)/(mRNA levels in control LN) × 100. (C) Kaplan-Meier survival curves of C57BL/6 (n = 93), BALB/c (n = 30), and D_HLMP2A with (n = 12) or without (n = 36) CLL treatment, and μMT with (n = 15) or without (n = 50) CLL treatment prior to footpad infection with VSV-IND. D_HLMP2A versus D_HLMP2A + CLL: p < 0.0001; μMT versus μMT + CLL: not significant; μMT versus D_HLMP2A + CLL: not significant; Log-rank (Mantel-Cox) test.

Figure 5. LT Signaling Is Required for SCS Macrophage Differentiation but Is Dispensable for LN Viral Capturing

(A–F) Representative confocal micrographs of LN sections from control (A, C, E) and LTβR-Ig-treated (B, D, F) C57BL/6 mice, stained with MAbs against CD169 (green) and SIGN-R1 (red). Panels (C) and (D) are high-magnification images of SCS regions, and (E) and (F) are high-magnification images of medullary regions. Scale bars represent 100 μm (A, B) and 20 μm (C–F). (G) Viral titers in popliteal LNs of C57BL/6, μMT, and LTβR-Ig-treated C57BL/6 mice, 30 min after footpad VSV infection. Results are representative of two experiments (n = 3–4 per experiment). Differences among means are not statistically significant.

Figure 6. LT Signaling Is Required for LN VSV Replication and IFN-I Production

(A–D) Representative confocal micrographs of LN sections from control (A, C) and LTβR-Ig-treated (B, D) C57BL/6 mice at 8 hr after footpad infection with VSV-eGFP (green). Sections were stained with MAbs against CD169 (red) and B220 (blue). Scale bars represent 150 μm (A, B) and 100 μm (C, D). (E) Viral titers in popliteal LNs of C57BL/6, μMT, and LTβR-Ig-treated C57BL/6 mice, 8 hr after footpad VSV infection. Results are representative of three experiments (n = 4 per experiment). ***p < 0.001. (F and G) Ifnb1 (F) and Ifna2 (G) mRNA levels in total RNA isolated from LNs of μMT and C57BL/6 mice that were treated with CLL or with LTβR-Ig for 1 or 2 weeks prior to infection with VSV-IND. Mice were sacrificed 8 hr p.i. Analysis was performed as indicated in Figures 1D and 1E. Graphs are representative of two experiments (n = 3–4 per experiment). ***p < 0.001; **p < 0.01; *p < 0.05.

Figure 7. B Cell-Derived LTα1β2 Is Required for Survival upon Subcutaneous VSV Infection, although Adaptive Immunity Is Dispensable

(A) Kaplan-Meier survival curves of footpad VSV-infected C57BL/6 (treated or not with LTβR-Ig), μMT, or irradiated C57BL/6 mice that were reconstituted with Lta^–/–, B Ltb^–/–, or T Ltb^–/– bone marrow (BM). C57BL/6 versus B Ltb^–/– BM → C57BL/6, p < 0.0001; C57BL/6 versus T Ltb^–/– BM → C57BL/6, not significant; B Ltb^–/– BM → C57BL/6 versus C57BL/6 + LTβR-Ig, not significant; B Ltb^–/– BM → C57BL/6 versus μMT, not significant; B Ltb^–/– BM → C57BL/6 versus Lta^–/– BM → C57BL/6, not significant; Log-rank (Mantel-Cox) test. (B) Kaplan-Meier survival curves of C57BL/6, (n = 10) BALB/c, (n = 10), and μMT (n = 13) D_HLMP2A mice that were untreated (n = 11) or treated with CLL alone (n = 11), or Thy1 antibody alone (n = 15) or with CLL and Thy1 antibody (n = 15) prior to footpad VSV infection. BALB/c versus D_HLMP2A + α-Thy1, not significant; BALB/c versus D_HLMP2A + α-Thy1 + CLL, p < 0.0001; D_HLMP2A + α-Thy1 + CLL versus μMT, not significant; D_HLMP2A + α-Thy1 + CLL versus D_HLMP2A + CLL, not significant; Log-rank (Mantel-Cox) test.