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Journal of Virology, March 2008, p. 3135-3138, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.01727-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Differential Requirements for T Cells in Viruslike Particle- and Rotavirus-Induced Protective Immunity

Sarah E. Blutt,^1,2 Kelly L. Warfield,^1, Mary K. Estes,¹ and Margaret E. Conner^1,2*

Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030F,¹ Michael E. Debakey Veterans Affairs Medical Center, Houston, Texas 77030²

Received 8 August 2007/ Accepted 19 December 2007

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Correlates of protection from rotavirus infection are controversial. We compared the roles of B and T lymphocytes in protective immunity induced either by intranasally administered nonreplicating viruslike particles or inactivated virus or by orally administered murine rotavirus. We found that protection induced by nonreplicating vaccines requires CD4⁺ T cells and CD40/CD40L. In contrast, T cells were not required for short-term protective immunity induced by infection, but both T-cell-dependent and -independent mechanisms contributed to long-term maintenance of protection. Our findings indicate that more than one marker of protective immunity exists and that these markers depend on the vaccine that is administered.

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Rotavirus causes dehydrating diarrhea in children, resulting in over 600,000 deaths worldwide each year (21). Correlates of protection from rotavirus infection are controversial. Immunoglobulin A (IgA) levels moderately correlate with protection from severe disease in humans (14, 17, 24), but studies with the extensively used murine model of rotavirus infection indicate that IgA is dispensable for protection (20, 23). Both replicating homologous and heterologous rotaviruses and nonreplicating protein and subunit vaccines induce protection in mice against a rotavirus challenge (7). Complete protection against reinfection is induced by replicating murine rotaviruses in the absence of T cells (10), while protection induced by vaccination with the capsid protein VP6 depends solely on the presence of CD4⁺ T cells (16). These data suggest that although antigenically similar, replicating virus and nonreplicating protein vaccines trigger different pathways of protective immunity against rotavirus infection with differential requirements for T cells. Here, we examine the contributions of B and T lymphocytes to protective immunity induced by the intranasal administration of nonreplicating viruslike particles (VLPs) or inactivated rotavirus and the oral administration of replication-competent wild-type murine rotavirus. Understanding and comparing the requirements for the induction of protective immunity against rotavirus infection will provide critical information to ascertain the correlates of protection from rotavirus infection.

Protection induced by a live viral infection, not VLPs, is maintained long-term. CD-1 mice (Charles River, Wilmington, MA) were vaccinated intranasally on days 0 and 14 with 10 µg of VLPs plus 5 µg of mutant Escherichia coli heat-labile toxin R192G (LT-R192G); they were orally challenged after 6 weeks with the wild-type murine strain of rotavirus (EC_wt), and the percentage of protection was calculated (1, 6, 8). High levels of protection (60 to 100%) are achieved 6 weeks after the administration of rotavirus VLPs composed of proteins VP2 and VP6 (2/6-VLPs) but are significantly lower than the levels of protection induced by EC_wt infection (18, 19). Orally administered EC_wt induces complete protection from infection (100%) at 6 weeks that is maintained at 6 months (11). To determine if VLP-induced protection persists beyond 6 weeks, mice were vaccinated with VLPs and challenged with EC_wt 6 months later. As expected, the mice vaccinated with VLPs exhibited a significantly lower level of protection than the mice that received a primary EC_wt infection (Fig. 1). Unlike what has been observed with a soluble recombinant VP6 protein vaccine (16), the level of protection induced by VLPs was not maintained over time, as it was significantly lower after 6 months than it was after 6 weeks (Fig. 1). This could be attributed to inherent differences between the soluble recombinant protein vaccine and the subunit particulate vaccine or to the differences in the strains of murine rotavirus used as a challenge. Unlike VLP-induced protection, the high level of protection induced by EC_wt infection was maintained over time (Fig. 1). Therefore, VLP-mediated protection results from the induction of pathways different from those induced by a live viral infection.

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FIG. 1. VLP-induced immunity against rotavirus infection is not maintained long-term. CD-1 mice were vaccinated intranasally with 2/6-VLPs (VLP) on days 0 and 14 or administered 105 50% infective doses (ID50) of ECwt (RV) on day 0. Mice were challenged on day 42 (short-term [ST]) or at 6 months (long-term [LT]) with 105 ID50 of ECwt, and the percentages of protection were assessed. The mean values for each group are indicated by the bars and the percentages (five mice per group). *, P < 0.05, compared to the results for the ST mice vaccinated with VLPs as determined by the Mann-Whitney U test; #, P < 0.05, compared to the results for the respective RV group as determined by the Mann-Whitney U test.

B cells contribute to, but are not essential for, protection from rotavirus infection. To assess the contribution of B cells to rotavirus protective immunity, JhD^–/– mice [C;129(B6)-IgH-J^tm1Dhu N?+N2] intercrossed to homozygosity on a BALB/c background (Taconic, Germantown, NY) or BALB/c mice were vaccinated or administered EC_wt. Mice were also vaccinated intranasally with 10 µg of inactivated rhesus rotavirus (4) plus adjuvant to address whether replication was an important component in the induction of protective immunity. Vaccinated JhD^–/– mice exhibited significantly lower levels of protection than vaccinated BALB/c mice (Fig. 2A). As reported previously for C57BL/6 mice expressing the JhD mutation (9, 15), protection established in BALB/c mice expressing the JhD^–/– mutation by EC_wt infection was less than that established in BALB/c mice (Fig. 2B). However, protection induced by EC_wt infection in JhD^–/– mice was significantly higher than that induced in VLP-treated or inactivated-virus-treated JhD^–/– mice (Fig. 2B). In agreement with previous findings (12), protection induced by EC_wt was not maintained over time, as JhD^–/– mice challenged at 6 months exhibited significantly lower levels of protection than mice challenged at 6 weeks (Fig. 2B). These findings indicate that B cells contribute to the induction of complete protection from rotavirus infection in BALB/c mice but that substantial protection (>60%) persists after 6 months in the absence of antibody.

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FIG. 2. B cells contribute to the protective immunity induced by a live viral infection or by nonreplicating vaccines. (A) Wild-type BALB/c and B-cell-deficient JhD mice were vaccinated intranasally with 2/6-VLPs (VLP) or inactivated rotavirus (InV) on days 0 and 14 and challenged with 106 50% infective doses (ID50) of ECwt on day 42. Daily stool samples were analyzed by enzyme-linked immunosorbent assay for rotavirus antigen. The results are displayed as percentages of protection for the individual animals, and the mean values for each group are indicated by the bars and percentages (five mice per group). *, P < 0.05, compared to the results for the identically treated BALB/c mice as determined by the Mann-Whitney U test. (B) BALB/c and JhD mice were administered 106 ID50 of ECwt on day 0 and challenged with 106 ID50 of ECwt either on day 42 (short-term [ST]) or at 6 months (long-term [LT]). *, P < 0.05, compared to the results for the ST JhD mice as determined by the Mann-Whitney U test; #, P < 0.05, compared to the results shown in panel A for the VLP- or InV-vaccinated JhD–/– mice as determined by the Mann-Whitney U test.

T cells are essential for protection induced by vaccines and long-term maintenance of protection. Although T cells are not required for protection induced by rotavirus infection (10), T cells are essential for protection induced by a soluble VP6 vaccine (5, 16). To further characterize the contribution of T cells to protection from rotavirus infection, β/ T-cell-receptor knockout (TCRKO) mice (B6.129P2-Tcrb^tm1Mom Tcrd^tm1Mom/J; Jackson Laboratory, Bar Harbor, ME) or the Jackson Laboratory-recommended control C57BL/6 mice were vaccinated with VLPs or inactivated virus or were orally administered EC_wt. Vaccinated TCRKO mice failed to develop detectable fecal or serum antibody, while infected TCRKO mice had detectable but significantly lower titers of both fecal and serum rotavirus-specific antibody than the C57BL/6 mice (data not shown). Vaccinated TCRKO mice exhibited limited protection from EC_wt 6 weeks after vaccination compared to that exhibited by the C57BL/6 mice (Fig. 3A). Virus-infected TCRKO mice were highly protected against a challenge at 6 weeks, but protection waned significantly by 6 months (Fig. 3B). Antibody titers were similar at 6 weeks and 6 months, indicating that the difference in protection was not due to waning antibody levels (data not shown). These results suggest that T cells are essential for protection induced by nonreplicating vaccines and contribute to the maintenance of long-term protection through a non-antibody-mediated process induced by rotavirus infection in C57BL/6 mice.

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FIG. 3. T cells are essential for protection induced by nonreplicating vaccines but only contribute to protection induced by a live viral infection. (A) Wild-type C57BL/6 or TCRKO mice were vaccinated intranasally with 2/6-VLPs (VLP) or inactivated rotavirus (InV) on days 0 and 14 and challenged on day 42 with 103 50% infective doses (ID50) of ECwt. Daily stool samples were analyzed by enzyme-linked immunosorbent assay for rotavirus antigen. The results are displayed as percentages of protection for the individual animals, and the mean values for each group are indicated by the bars and percentages (five mice per group). *, P < 0.05, compared to the results for the identically treated C57BL/6 mice as determined by the Mann-Whitney U test. (B) C57BL/6 and TCRKO mice were administered 103 ID50 of ECwt on day 0 and challenged with 103 ID50 of ECwt either on day 42 (short-term [ST]) or at 6 months (long-term [LT]). *, P < 0.05, compared to the results for the ST TCRKO mice as determined by the Mann-Whitney U test.

CD4⁺ T cells and the expression of CD40/CD40L are essential for protection induced by nonreplicating vaccines. We investigated whether protection could be attributed to either the CD4 or CD8 T-cell subsets. CD4^–/– (B6.129S2-CD4^tm1Mak/J) and β₂m^–/– (B6.129P2-B2 m^tm1Unc/J) mice (both on a congenic C57BL/6 background; Jackson Laboratory) were vaccinated with 2/6-VLPs or orally administered EC_wt. The CD4^–/– and β₂m^–/– mice infected with EC_wt were completely protected from a rotavirus challenge at 6 weeks (Fig. 4). The vaccinated β₂m^–/– mice exhibited high levels of protection (89%); however, these levels of protection were significantly lower than those induced by EC_wt infection (Fig. 4). The vaccinated CD4^–/– mice had significantly lower levels of protection and geometric mean titers of fecal antibody than the vaccinated wild-type β₂M^–/– mice and the EC_wt-infected CD4^–/– mice (Fig. 4 and data not shown). Therefore, protection induced by 2/6-VLPs appears to be modulated by CD4⁺ T cells, with a small contribution from CD8⁺ T cells.

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FIG. 4. T-cell-mediated protection induced by nonreplicating vaccines requires CD4 but not CD8 T cells. β2m–/–, CD4–/–, CD40–/–, and CD40L–/– mice were vaccinated with 2/6-VLPs (VLP) on days 0 and 14 or administered 103 50% infective doses (ID50) of ECwt (RV). The mice were challenged on day 42 with 103 ID50 of ECwt, and daily stool samples were analyzed by enzyme-linked immunosorbent assay for rotavirus antigen. The results are displayed as percentages of protection for the individual mice, and the bars and percentages indicate the means for each group (five mice per group). *, P < 0.05, compared to the results for the C57BL/6 mice shown in Fig. 3 as determined by the Mann-Whitney U test; #, P < 0.05, compared to the results for the respective strain administered ECwt as determined by the Mann-Whitney U test.

T cells mediate specific antibody production through CD40L signaling (13). To determine if CD40/CD40L expression is critical to VLP- or infection-induced protective immunity, CD40L^–/– (B6.129S2-CD40lg^tm1Imx) and CD40^–/– (B6.129P2-CD40^tm1Kik/J) mice (both congenic on a C57BL/6 background; Jackson Laboratory) were vaccinated with 2/6-VLPs or administered EC_wt. The vaccinated CD40^–/– or CD40L^–/– mice failed to develop fecal or serum antibody (data not shown) and had much lower levels of protection against rotavirus challenge at 6 weeks (Fig. 4) than the C57BL/6 mice (Fig. 3B) and the CD40^–/– and CD40L^–/– mice receiving the EC_wt infection (Fig. 4). In contrast, all infected mice developed serum and fecal antibody titers similar to those of TCRKO and CD4^–/– mice, which were significantly reduced compared to those of the wild-type mice (data not shown), and were completely protected after 6 weeks (Fig. 3B and 4). These data indicate that CD40/CD40L signaling is an important component of nonreplicating-vaccine-induced protective immunity to rotavirus infection. Although CD40/CD40L plays an essential role in antibody production, the interaction also mediates antigen-presenting cell activation and influences CD4 or CD8 cellular immunity (22). More studies are necessary to determine whether one or both of the CD40/CD40L signaling functions are critical to VLP-induced immunity.

The contribution of T cells to rotavirus protective immunity. Clearly, subunit vaccines induce protection that requires CD4⁺ T cells. Neither the VLPs or inactivated virus examined above nor a soluble VP6 vaccine (16) induce protection in the absence of CD4⁺ T cells. In contrast, CD4⁺ T cells contribute little to short-term protective immunity induced by replicating virus, which supports previous work indicating that rotavirus infection induces a strong T-cell-independent response (2, 10). However, after examination of the maintenance of protection in the absence of T cells, we find that T cells support and enhance long-term immunity against rotavirus infection. Interestingly, although the lack of T cells causes a decrease in protective immunity, substantial protective immunity is still present after 6 months, indicating that T-cell-independent immunity persists longer than anticipated. Therefore, rotavirus immunity is induced through both T-cell-dependent and -independent pathways. One caveat to examining the effect that lymphocyte subsets have on the induction of rotavirus protective immunity is the influence of the genetic background on the mutation phenotype. The ability of SCID mice to clear rotavirus infection differs based on whether the defect is present in BALB/c or C57BL/6 mice (10). Our own work has indicated significant differences between the two strains in susceptibility to rotavirus infection, as well as in protection, induced by VLPs (3). More studies are necessary to tease out the specific features that are unique to the inbred mouse strains that influence rotavirus immunity.

 
We thank John Clements for providing the LT-R192G; David Keeland, Ann Robertson, Natalie Dang, Jillian Pennington, Erin Sargent, and John Dawson for technical assistance; and the Texas Gulf Coast Digestive Disease Center for helpful advice.

This work was supported by NIH AI10604 (S.E.B.), NIH AI24998 (K.L.W., M.K.E., and M.E.C.), and NIH DK56338 (M.K.E.) and by the Office of Research and Development, Medical Research Service, Department of Veterans Affairs (S.E.B. and M.E.C.).

 
* Corresponding author. Mailing address: Department of Molecular Virology and Microbiology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030. Phone: (713) 798-3590. Fax: (713) 798-3586. E-mail: mconner{at}bcm.edu

Published ahead of print on 9 January 2008.

Present address: United States Army Medical Research Institute of Infectious Diseases, 1425 Porter Street, Fort Detrick, MD 21702.

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Journal of Virology, March 2008, p. 3135-3138, Vol. 82, No. 6
0022-538X/08/$08.00+0     doi:10.1128/JVI.01727-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Blutt, S. E. Articles by Conner, M. E. Search for Related Content PubMed PubMed Citation Articles by Blutt, S. E. Articles by Conner, M. E.