With an increasing incidence of severe disease and increased rates of hospitalization in both infants and the elderly, the development of a safe and effective vaccine against RSV is of tremendous importance. However, an incomplete understanding of RSV pathogenesis and the failed vaccine trials of the 1960s hinder the accomplishment of this goal. With its role as the viral attachment protein (31
) and as the source of much of the sequence variation between virus strains (20
), it is desirable to include the G glycoprotein as a component of any RSV immunogen. However, the similar patterns of severe disease induced by immunization with FI-RSV or with RSV G complicate the question of the safety of a vaccine construct containing RSV G. While disease in FI-RSV- and G-immunized animals may appear to be analogous, there are several indications that they have distinct pathogenic mechanisms that lead to a common final pathway resulting in enhanced disease.
It has been clearly demonstrated that both immunization with FI-RSV (9
) and immunization with RSV G (4
) predispose for severe disease typified by severe illness, pulmonary eosinophilia, and type 2 cytokine production. However, although the endpoints may be similar, it is apparent that the cytokine requirements for these two immunogens are distinct. While illness and type 2 cytokine production are reduced in RSV-challenged FI-RSV-immunized mice with IL-4 depletion (23
), the enhanced disease is unaltered in G-primed mice when IL-4 function is inhibited either by antibody depletion or in IL-4-deficient mice (23
). Furthermore, inhibition of IL-13 alone modulates disease only in FI-RSV-primed mice (25
). Both IL-4 and IL-13 function must be blocked to modulate disease in mice immunized with RSV G (25
). Additionally, when RSV glycoprotein F is administered as a purified protein in the context of alum, immune responses that result in eosinophilia and IL-4 and IL-5 production following RSV challenge are induced (8
). However, in contrast to G-specific responses, F-specific immune responses may be modified by Th1-modulating adjuvants such as monophosphoryl lipid A or QS-21 (17
). Thus, the ability to induce disease-enhancing immune responses is not restricted to RSV G but appears to be a consequence of parenteral administration of RSV antigen and presentation to the immune system as a soluble protein that cannot be processed via the major histocompatibility complex class I pathway.
Vaccine-enhanced disease was observed in approximately 80% of FI-RSV vaccinees in the 1960s trial (26
) and occurs in many animal models (5
). In contrast, RSV G-induced immune responses associated with vaccine-enhanced illness exhibit some degree of genetic restriction, with pulmonary eosinophilia being absent (19
) or dramatically reduced (23
) in vvGs-immunized mouse strains other than BALB/c. The peptide at aa 183 to 198 of the RSV G glycoprotein is sufficient to elicit pulmonary eosinophilia and both type 1 and type 2 cytokine production in BALB/c mice (44
) and is largely restricted to a subset of CD4+
T cells expressing the Vβ14 TCR (54
). The existence of this immunodominant epitope that alone is sufficient to induce those immune responses associated with vaccine-enhanced disease underscores the phenomenon of genetically restricted RSV G immunogenicity, which is not consistent with the nearly universal induction of vaccine-enhanced illness by the FI-RSV vaccine in children less than 6 months of age (28
These observations, together with the distinct cytokine requirements in FI-RSV- and vvGs-immunized mice, led us to hypothesize that while Vβ14+
T cells mediate much of the eosinophil recruitment and cytokine production in vvGs-primed RSV-challenged mice, such an oligoclonal T-cell subset would not be expanded and would not significantly contribute to vaccine-enhanced disease in FI-RSV-immunized mice. As reported previously (54
), these data confirm that the depletion of Vβ14+
T cells at the time of challenge of vvGs-immunized mice decreases the degree of pulmonary eosinophilia and type 2 cytokine production. In marked contrast, however, anti-Vβ14 treatment of FI-RSV-immunized mice has little impact on any aspect of disease examined in these studies. Furthermore, analyses of TCR expression and T-cell specificity underscore the distinct differences in T-cell subsets. Whereas vvGs-immunized mice show a prevalence of Vβ14+
T cells that are specific for a single immunodominant epitope encompassing aa 183 to 195 of RSV G, RSV challenge of FI-RSV-immunized mice amplifies CD4+
T cells expressing a diverse Vβ TCR repertoire with no predominance for an individual peptide apparent. This finding suggests that T cells induced by FI-RSV immunization are specific for an as-yet-unidentified RSV peptide or that they are specific for a diverse array of peptides.
It is not necessarily unexpected to see a selected expansion of a particular Vβ-restricted T-cell subset upon exposure to RSV G. Pathogenic roles for specific T-cell subsets in several models of infection and in various autoimmune diseases, including a disease-causing population of Vβ14+
T cells in a mouse model of ulcerative colitis (36
), have been described (6
). Several of those studies describe a protective effect of selected depletion of the particular T-cell subset associated with disease without any toxicity or observable effect in normal populations (6
). In fact, the deletion of the disease-causing T-cell subsets is an effective treatment for at least two autoimmune syndromes in humans (6
). Thus, while studies have not described the effects of Vβ14+
-T-cell depletion in normal uninfected mice, these studies and the data reported here (for RSV-challenged mice primed with vac-lac or FI-RSV) would suggest that the elimination of this T-cell subset does not adversely affect the general immune status of mice.
These data demonstrate that distinct immune responses mediated by discrete T-cell subsets result in pulmonary eosinophilia and type 2 cytokine production in vvGs- and FI-RSV-immunized mice. Thus, the induction of a single oligoclonal G-specific CD4+-T-cell subset is not the basis for vaccine-enhanced disease induced by FI-RSV immunization. Therefore, these data suggest that RSV G may be safely included in a vaccine product if the potential vaccine is properly formulated to target RSV antigens to endogenous antigen-processing and presentation pathways. Furthermore, these data confirm that a restricted T-cell subset mediates RSV G-specific immune responses in BALB/c mice. In contrast, within the parameters utilized in this study, these data do not support the hypothesis that FI-RSV immunization induces oligoclonal T-cell subsets with restricted peptide specificities and suggest that either the peptide specificity in FI-RSV-immunized mice has yet to be identified or that FI-RSV-induced responses have diverse specificities. These observations may suggest that FI-RSV vaccine-enhanced disease will be produced in nearly all populations with diverse genetic backgrounds. In contrast, the severe disease of RSV G-immunized RSV-challenged mice, which is clearly genetically restricted, may be comparable to the severe disease of children during primary infection. Information gained from an investigation into the pathogenic mechanisms of RSV G-induced disease may provide insights into the immunologic basis for severe primary RSV disease in humans, while further research is necessary to understand the underlying mechanisms of FI-RSV vaccine-enhanced disease and to aid in the proper formulation of a safe vaccine product.