Generation of protective T cell-independent antiviral antibody responses in SCID mice reconstituted with follicular or marginal zone B cells

Abstract

B cells generated in the bone marrow of adult mice enter the periphery as transitional B cells and subsequently differentiate into one of two phenotypically and functionally distinct subsets, marginal zone (MZ) or follicular (Fo) B cells. Recent reports indicate, however, that in response to environmental cues, such as lymphopenia, mature Fo B cells can change to display phenotypic markers characteristic of MZ B cells. Previously, we found that splenic B cells transferred to SCID mice responded to polyoma virus (PyV) infection with T cell-independent (TI) IgM and IgG secretion, reducing the viral load and protecting mice from the lethal effect of the infection. The contribution of MZ and Fo B cell subsets to this antiviral TI-2 response, however, has not been addressed. In this study, we show that both sort-purified MZ and Fo B cells generate protective TI Ab responses to PyV infection when transferred into SCID mice. Moreover, the transferred Fo B cells in the spleens of the PyV-infected SCID mice change phenotype, with many of them displaying MZ B cell characteristics. These findings demonstrate the plasticity of the B cell subsets in virus-infected hosts and show for the first time that B cells derived exclusively from Fo B cells can effectively function in antiviral TI-2 responses.

FIGURE 1

MZ and Fo B cells transferred into SCID mice generate TI Ab responses to PyV. A, MZ (B220⁺CD21^highCD23^low) and Fo (B220⁺ CD21^lowCD23^high) B cells were sorted from B6 splenocytes by FACS; the isolated populations are shown. B, VP1-specific ELISA of serum IgM and IgG of SCID mice reconstituted with 2 × 10⁶ MZ B cells (n = 5), 2 × 10⁶ Fo B cells (n = 5), or 1 × 10⁷ Fo B cells (n = 9) and infected with PyV. The sera for IgM was taken on day 14 and for IgG on day 21 after infection and tested at 1/100 dilutions. Sera of uninfected B6 mice were used as negative controls. Mean absorbance at 450 nm with SEM are shown. Data are representative of three experiments.

FIGURE 2

MZ and Fo B cells protect SCID mice from death following PyV infection. SCID mice were reconstituted with 2 × 10⁶ MZ B cells, 2 × 10⁶ Fo cells, 1 × 10⁷ Fo B cells, or no cells at all, and 1 day later infected with 2 × 10⁶ PFU of PyV i.p. The mice were observed for 90 days.

FIGURE 3

Changes in the MZ and Fo B cell subsets following transfer to SCID mice and PyV infection. Phenotype of B cells in the spleens of naive B6 mice (A) and SCID mice reconstituted with B cells isolated from spleens of B6 mice (10⁷cells/mouse) and left uninfected (B) or infected for 22 days with PyV (C–F). The FACS plots were gated on CD19⁺B220⁺ lymphoid cells. The percentage of CD21^highCD23^low and CD21^low CD23^highB cells are shown in each sample.

FIGURE 4

Phenotype of B cells in the spleens of mice reconstituted with FACS-sorted Fo B cells. CD21^lowCD23^high Fo B cells were sorted from spleens of B6 Ly5.1-congenic mice to ~96% purity and 10⁷ cells were transferred into Ly5.2 SCID hosts i.v. Mice were infected the next day with 2 × 10⁶ PFU of PyV i.p. or left uninfected. The FACS plots shown were gated on Ly5.1 (donor) CD19⁺B220⁺ cells. A, Unsorted spleen cells of B6 mice; B, sorted Fo B cells used for transfer; C–E, spleen cells of SCID mice 17 days after reconstitution and PyV infection; F–H, spleen cells of uninfected SCID mice 17 days after reconstitution. I and J, CD21^lowCD23^highAA4.1⁻ Fo B cells were sorted from B6 Ly5.1 spleens (99.6% purity) and 7.5 × 10⁶ cells were transferred into Ly5.2 SCID mice, then the mice were infected with 2 × 10⁶ PFU of PyV i.p. on the next day. Spleen cells were analyzed by FACS on day 14 after infection. The FACS plots are gated on Ly5.1 CD19⁺B220⁺ cells. The data represent one of two similar independent experiments.