Our results show that anti-T1/ST2 treatment reduces the severity of Th2-mediated eosinophilic immunopathology in the BALB/c mouse model of RSV infection. Treatment decreased weight loss, local recruitment of inflammatory cells (including eosinophils), and lung lavage levels of IL-5, TNF, and IFN-γ in RSV-infected mice presensitized with the attachment protein G. However, no benefit of anti-T1/ST2 treatment was found in the noneosinophilic immunopathology seen in F/RSV mice or in G/RSV mice previously treated with IL-12.
Previous studies show that T1/ST2 is preferentially expressed on T cells that predominantly produce IL-4, IL-5, or IL-10 but not IFN-γ or IL-2 15
. Expression is found in Th2-induced granulomatous lung disease of Schistosoma mansoni
, and in vitro studies demonstrate upregulation of T1/ST2 under Th2-polarizing conditions 11
. Signaling through T1/ST2 after adoptive transfer of OVA-treated dendritic cells is essential for sensitization to inhaled OVA and development of Th2-dependent airway eosinophilia 37
. In this study, T1/ST2 expression was only observed in G protein–sensitized (eosinophilic) mice and not in those previously sensitized with the fusion protein or those experiencing a primary RSV infection (both noneosinophilic). These results, together with the observation that IL-12 treatment reduces T1/ST2 expression in G/RSV-infected mice, support the Th2-restricted nature of this molecule.
The ability of anti-T1/ST2 antibody treatment of G/RSV-infected mice to abrogate T1/ST2 expression on BAL cells does not clarify whether T1/ST2+
cells are depleted or the receptor merely masked by the antibody. The effect on Th2-mediated responses was striking, however. Anti-T1/ST2 treatment was accompanied by a reduction in IL-5, TNF, and IFN-γ but not IL-4 levels in the lung lavage of G/RSV-infected animals. The reduction in pulmonary eosinophilia may be explained by reduced IL-5 and is similar to that reported in T1/ST2 knockout mice infected with S. mansoni
. The reduction in both Th1 and Th2 cytokines is intriguing considering that reduced IL-4 and IL-5 but increased IFN-γ is observed when OVA-specific Th2 cells are incubated with a T1/ST2 Ig fusion protein in vitro 11
. When OVA-specific cells are adoptively transferred, however, T1/ST2 depletion reduces Th2 cytokine levels in the lung lavage but does not significantly alter IFN-γ. Mediastinal lymph node cells from T1/ST2 knockout mice produce less IL-4 and IL-5 and increased IFN-γ in response to S. mansoni
. The discrepancies between our results and those of others are likely to reflect the nature of the antigen (i.e., replicating versus nonreplicating) and the infected cell type. Our results also suggest that removal of T1/ST2-expressing cells does not necessarily enhance Th1 responses. The G protein only induces CD4+
T cells and, in the absence of CD8+
T cells, these assume a Th2 cytokine profile 36
. As anti-T1/ST2 treatment was performed during the RSV challenge (not during recombinant vaccinia priming), the induction of a replacement Th1 population would not be anticipated. The lack of an effect on IL-4 in lavage fluid of anti-T1/ST2–treated, G/RSV-infected mice may indicate IL-4 production by an alternative cell type. CD4+
cells secrete IL-4 but are T1/ST2− 15
and would therefore not be inhibited in our experimental design.
The reduced weight loss in the G/RSV group treated with anti-T1/ST2 may be linked to lower TNF levels. TNF has multiple actions and plays a critical role in inflammation. TNF also plays an important role in recruitment of cells to normal and inflamed tissues 3940
and may induce early neutrophil and eosinophil recruitment 41
. When released in large quantities, it enters the bloodstream and is associated with weight loss and cachexia. TNF production is seen predominantly, but not exclusively, in T1/ST2−
T cells 14
, and the reduced levels of this cytokine may be due in part to the decreased total inflammatory cell recruitment seen with T1/ST2 depletion in this study. A reduction in S. mansoni
–induced primary granuloma volume in T1/ST2 knockout mice 24
is also consistent with the reduced inflammatory infiltrate seen in our study. No previous reports show a decrease in TNF by T1/ST2 inhibition. T1/ST2 depletion did not inhibit illness in G/RSV-infected mice treated with IL-12 or those previously sensitized with the F protein. As IL-12 treatment reduces T1/ST2+
cells and such cells are scarce in F protein–sensitized mice, anti-T1/ST2 treatment does not alter total cell recruitment to the lung and weight loss remains unaltered.
An enhanced T cell–dependent antibody response occurs in G/RSV-infected mice and promotes rapid viral clearance compared with mice experiencing a primary infection (β gal/RSV). Considering the reduced T cell recruitment to the lung, it is not surprising that anti-T1/ST2 treatment decreases RSV-specific antibody production and delays viral clearance. To our knowledge, there are no previous reports of delayed clearance of a pathogen due to anti-T1/ST2 treatment. As illness in this model does not depend on uncontrolled viral replication but rather on the extent of cell recruitment, the beneficial effects of T1/ST2 inhibition are not offset by the delayed viral clearance. A reduction in antibody has previously been observed in OVA-challenged mice after T1/ST2 depletion 11
but not in T1/ST2 knockout animals infected with S. mansoni 24
Studies of T1/ST2-knockout mice have produced conflicting results. One group reported that T1/ST2-deficient animals fail to show decreased Th2 cytokine or Ig production in response to Nippostrongylus braziliensis
infection or allergen-induced airway inflammation 25
. An independently derived T1/ST2 knockout model, however, shows that basal immunological functions are not affected but that Th2 cytokine responses are impaired in a fashion similar to that seen in depletion studies. The findings in the latter study suggest that this molecule plays an important role in the early events of Th2 development, as primary but not secondary granuloma formation was of decreased magnitude 24
. In another study, both a T1/ST2-deficient mouse strain and a transgenic mouse strain expressing high levels of an extracellular T1/ST2 domain–Fc fusion protein (acting as T1/ST2 ligand scavenger) cleared N. brasiliensis
infection as efficiently as control mice. Lung lymphocytes, however, did produce less IL-5, and these mice had a slightly diminished recruitment of eosinophils into the lung 26
. As T1/ST2 is not only expressed on CD4+
T cells but also on mast cells and fibroblasts 42
, the knockout mice may not only reflect T helper cell phenotype modulation but also harbor other developmental changes.
Inhibition of specific Th cell phenotypes is a promising approach to the treatment of various inflammatory conditions. Th2 inhibition may be beneficial in both prevention and management of allergic inflammation, but to date treatments have not been able to reliably counter Th2-driven illness in the clinical setting. T1/ST2 neutralization may offer a new therapeutic opportunity, but more information is needed to define its role. In these studies, we have been able to demonstrate a beneficial role for anti-T1/ST2 antibody treatment in selective Th2-directed immunopathology in G-primed mice during RSV infection. Our results offer the prospect that selective anti-T1/ST2 treatment will be beneficial in Th2-driven conditions.