The major goal of this study was to examine directly the capacity of Th1 cells to counterbalance the proasthmatic effects of Th2 cells in a murine model of asthma. The Th1/Th2 paradigm suggests that Th1 and Th2 cells counterbalance each other and that Th1 cells protect or prevent Th2-mediated allergic disease and asthma (27
). However, using well-defined, phenotypically committed OVA-specific Th1 and Th2 cells expressing identical TCRs and adoptively transferred into either SCID mice or into OVA-immunized BALB/c mice, we found that OVA-specific Th1 cells failed to reverse Th2-mediated airway inflammation and airway hyperreactivity, even when given in twofold excess. These studies indicate that the Th1/Th2 paradigm, which predicts that Th1 cells downregulate allergic disease and asthma, may be more complex than initially appreciated and that suppression of allergic inflammation and Th2 activity in vivo
may depend on cells other than Th1 lymphocytes.
Inasmuch as asthma is associated with the presence of lymphocytes producing Th2 cytokines (28
), it is not surprising that OVA-specific Th2 cells induced severe allergic inflammatory responses and airway hyperreactivity when adoptively transferred into normal (24
) or SCID recipients (Figs. and ). We also demonstrated, however, that antigen-specific Th0 cells caused severe airway inflammation and hyperreactivity equal to those observed with Th2 cells. This indicated that production of IFN-γ, a cytokine thought to inhibit the development of allergy and asthma (30
), by Th0 cells was insufficient to neutralize the effects of IL-4 and IL-5. Our studies differed from previous studies with Th2 cells (24
) in that we used SCID mice as recipients to eliminate the effects of host cells that could inhibit the function of the transferred Th cells or that could contribute to airway hyperreactivity on immunization with antigen. These results with SCID mice demonstrated that airway hyperreactivity can be induced by Th2 or Th0 cells in the absence of other T cells, B cells, IgE, or γδ T cells. In contrast, previous studies with B cell–deficient mice (32
) and γδ T cell–deficient mice (33
) suggested that B cells, IgE, and γδ T cells were required for the induction of airway hyperreactivity or allergic airway inflammation. It is possible, however, that B cells or γδ T cells are required for the development of Th2 effector cells from naive precursor cells but that once Th2 cells have become activated, such “accessory cells” are not essential for expression of airway inflammation and hyperreactivity.
Our results demonstrating the inability of Th1 cells to neutralize the effects of Th2 cells are surprising in view of numerous studies that suggested that allergen-specific Th1 cells might suppress allergic inflammation. For example, we and others have shown that conventional allergen immunotherapy, which improves symptoms in allergic and asthmatic patients, reduces IL-4 production (9
) and increases IFN-γ production in an allergen-specific fashion (10
). Immunization with IL-12–modified allergen (34
), with heat-killed Listeria monocytogenes
as adjuvant (35
), with intratracheal IL-12, or with naked DNA plasmids containing cDNA for allergens (36
) also resulted in a switch in cytokine production in allergen-specific CD4+
T cells and caused a reduction in allergen-induced airway hyperreactivity (38
). These observations suggest that allergen-specific Th1-polarized responses suppress allergic inflammation, although no previous studies have examined the capacity of Th1 cells to inhibit Th2-induced airway hyperreactivity directly.
On the other hand, there is increasing evidence that Th1 and Th2 cells may not always antagonize each other. First, IFN-γ has been identified in BAL fluid and serum of asthmatic patients, suggesting that Th1-like cells may in fact contribute to, rather than inhibit, the pathology in asthma (39
). In addition, Th1 cells are not found in large numbers in the lungs or mucus membranes of nonallergic or nonasthmatic individuals after allergen exposure, as would be predicted if Th1 cells reduced airway inflammation. Moreover, in a murine model of autoimmunity, myelin basic protein–specific Th2 cells, when transferred into immunodeficient mice, exacerbated rather than prevented experimental autoimmune encephalomyelitis (14
). Similarly, attempts to use autoantigen-specific Th2 cells to confer protection against Th1-induced diabetes mellitus unexpectedly produced intense and generalized pancreatitis and subsequent diabetes mellitus (15
). These studies together indicate that Th1 and Th2 cells may not always balance each other's function in a dichotomous paradigm, but rather may in some instances be unexpectedly harmful, not only in autoimmune disease but also in allergic disease and asthma.
We found that antigen-specific Th1 cells when adoptively transferred to SCID mice induced considerable airway inflammation, resembling severe delayed-type hypersensitivity reactions, lung allograft rejection (25
), or hypersensitivity pneumonitis (41
). The inflammatory response caused by Th1 cells depended on the presence of antigen and occurred only in the lungs of the recipient mice. Surprisingly, even though Th1 cells induced severe airway inflammation, they did not cause airway hyperreactivity. These studies indicate that inflammation in and of itself is not important for the development of airway hyperreactivity, although CD4+
T cells appear to be essential for the development of airway hyperreactivity (42
). The precise factors produced by CD4+
T cells that are important for the development of airway hyperreactivity, however, have not yet been identified, but they appear not to be IL-4 and IL-5 (42
) nor those produced by Th1 cells (Fig. ). Further studies are necessary to determine whether other factors produced by Th2 cells, such as IL-9 (43
) or IL-13 (44
), are essential for the induction of airway hyperreactivity.
Although we found that Th1 cells could not reverse Th2-induced airway hyperreactivity in SCID or in OVA-immunized BALB/c mice, Th1 cells were able to reduce the number of airway eosinophils and reduce intrabronchiolar mucus production induced by Th2 cells. Similarly, we reported previously (45
) that adoptively transferred Th1 clones could inhibit the capacity of Th2 clones to induce antigen-specific IgE synthesis. This indicated not only that the adoptively transferred Th1 cells functioned in vivo
after transfer and could modify some, although not all, functions of Th2 effector cells, but also that the presence of eosinophils may not be essential for the development of airway hyperreactivity, as has been suggested previously (46
). The resistance of Th2 cell function to modification by Th1 cells may be due in part to the fact that Th2 effector cells are terminally differentiated with fixed cytokine profiles (23
) and cease to express IL-12 and IL-18 receptors (47
It is possible that Th1 cells may in fact be beneficial in asthma if they are present early on in high numbers before activated effector Th2 cells are present. In such instances, while being ineffective in reversing the function of effector Th2 cells, Th1 cells may be capable of blocking the development of Th2 cells from naive T cells and of preventing the induction of airway hyperreactivity. Alternatively, other cell types — e.g.
, transforming growth factor-β–producing CD4+
antigen-specific Th3 cells (49
), Tr1 cells (50
), γδ cells (51
), or CD8+
) — may be more important than Th1 cells in downregulating allergic inflammation and hyperreactivity. Further studies are required to determine how Th1 cells or other cell types regulate allergic inflammation and airway hyperreactivity. Such studies are particularly important as prerequisites for the development of therapies focused on immunomodulation of deleterious Th2-polarized responses.
In summary, using well-defined polarized Th cells, we showed that antigen-specific Th1 cells were ineffective in reducing airway hyperreactivity induced by Th2 cells and caused serious airway inflammation but not airway hyperreactivity. In contrast, Th2 and Th0 cells induced considerable airway hyperreactivity, in the absence of B cells, IgE, or other lymphocytes. Our results raise concerns regarding the protective effects of Th1 cells in allergy and asthma and regarding the feasibility of Th1-based therapies for these clinical problems.