Viruses and Atopic Sensitization in the First Years of Life

FERNANDO D. MARTINEZ

Respiratory Sciences Center, University of Arizona, Tucson, Arizona

	INTRODUCTION

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

Reports indicating that in most cases of asthma the first symptoms of bronchial obstruction occur during the first years of life (1) have focused a great deal of attention on the environmental factors that may predispose to or protect against the development of the disease in early life. Because the great majority of wheezing lower respiratory illnesses (LRIs) in infancy and early childhood are associated with microbiological evidence of an acute viral infection (2), it is not surprising that there has been renewed interest in the role of viruses in the inception of asthma. It is now well established, on the other hand, that most cases of childhood asthma in school-age children are associated with evidence of sensitization to local aeroallergens (3). It is still unclear, however, whether sensitization to specific antigens is causally related to the development of asthma. In this article, the role of viral respiratory infections and of sensitization to aeroallergens as predisposing factors for asthma are assessed, with special reference to events occurring during the first years of life.

	EARLY VIRAL INFECTIONS AND ASTHMA

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

The role of viral infections in the development of asthma has been the matter of considerable research and debate. There is little doubt that many (if not most) asthmatic attacks occur in concomitance with microbiological evidence of viral infection (4). As stated earlier, most wheezing LRIs occurring during the first years of life are also caused by respiratory viruses. This has suggested the hypothesis that viruses may alter the development of the lungs or of the immune system during early life and may thus be important risk factors for the development of the disease. The empirical evidence regarding this hypothesis is not conclusive: Some studies (especially those based on infants hospitalized for bronchiolitis [5] or those in which follow-up did not go beyond the preschool years) show a strong association, while others have been unable to confirm this finding (6). Some years ago (7) we showed that children with wheezing LRIs in early life could be divided into two groups: those who still had reported wheezing episodes at the age of 6 yr ("persistent wheezers") and those whose wheezing episodes had remitted by that age ("transient wheezers"). We also showed that persistent wheezers had, as a group, many of the characteristics that would be expected in children at risk for asthma: higher levels of serum IgE, higher prevalence of maternal asthma, and higher incidence of atopic dermatitis, among others (7). Transient wheezers were not more likely to have any of these characteristics than children who never wheezed. More recently we have shown (8) that there are significant differences between transient wheezers and persistent wheezers in the acute reaction occurring at the time of the first LRI (at approximately 1 yr of age for both groups). Whereas persistent wheezers showed significant increases in total serum IgE at the time of the acute illnesses when compared with serum IgE levels measured in a sample obtained during the convalescent phase, transient wheezers showed no acute changes in total serum IgE during the LRI (8). Moreover, eosinophil counts were significantly lower at the time of the acute LRI when compared with eosinophil counts measured away from the acute LRI in both transient wheezers and in children who had LRIs without wheezing. No such eosinopenic responses could be observed in persistent wheezers (8). It is thus likely that differences in the nature of the immune response to viruses may already be present at the time of the first episode of airway obstruction in children who will go on to develop persistent wheezing later in life. It is interesting to stress here that there were no differences in the etiology of the first LRI between the two groups, with approximately two-thirds of all episodes in which viruses were isolated being due to respiratory syncytial virus (RSV) (8).

These results may offer a possible explanation for the different results obtained by different studies of association between wheezing LRIs in early life and the subsequent development of asthma. It is likely that these results will depend on the proportion of transient and persistent wheezers in the sample being studied. Our own results suggest, for example, that subjects with more severe symptoms in early life are overrepresented among persistent wheezers, and this may explain why studies based on subjects hospitalized with RSV in early life tend to show a stronger association with subsequent asthma (5). But these studies also suggest that it is not the RSV infection by itself that is responsible for the development of persistent wheezing episodes. More likely, it is a better understanding of the interaction between RSV or other respiratory viruses and the developing immune system that may offer important clues as to the nature of the connection between viral LRIs and the development of persistent wheezing episodes later in life.

Unfortunately, little is known about the nature of the immune response to RSV in the humans. Román and coworkers (9) showed that peripheral blood mononuclear cells (PBMCs) obtained from children hospitalized for RSV has both lower interleukin 4 (IL-4) and lower interferon  (IFN-) responses to nonspecific stimuli than PBMCs of control infants. However, the IFN-/IL-4 ratios were significantly higher in control than in infected children, suggesting that, although hospitalized infants with RSV may have deficits in both T helper type 1 (Th1) and Th2 responses, these deficits are more marked for the former than for the latter. Unfortunately, the authors could not identify the specific cells among PMBCs that were responsible for these observations, and thus the attribution of the observed deficits to Th cells was conjectural. Nevertheless, the data support our own finding that, among children at risk for developing asthma, an imbalance between IL-4 and IFN--producing cells may exist that explains the elevated IgE levels observed in persistent wheezers at the time of their first acute LRI.

	ANIMAL MODELS OF RSV INFECTION

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

Most of our knowledge regarding acute responses to RSV comes from experiments performed in laboratory animals, especially mice (10). It is now apparent that two main RSV gene products determine immune responses in these murine models: the G (or attachment) protein and the F (or fusion) protein. The G protein seems to elicit mainly a Th2-driven response characterized by lung eosinophilia and local production of IL-4 and IL-5, whereas the F protein stimulates a Th1-driven response with no lung eosinophilia (11). Considered in relation to our own findings (8) and those of Román and colleagues (9), it was plausible to surmise that animals in which the Th2-driven response was blocked by use of cytokines such as IL-12 would show decreased illness severity. Interestingly, blocking the Th2-driven response markedly decreased lung eosinophilia but was not associated with decreased severity of the local inflammatory process in mice (12). It is thus likely that, at least in this animal model, cells other than T helper lymphocytes may be important in determining the nature of the inflammatory response to RSV.

Studies by Hussell and coworkers (13, 14) suggest that this may indeed be the case. These studies have shown that natural killer (NK) cells are the main source of IFN- at the early phases of RSV infection in mice (14). Interestingly, NK cells, much like Th lymphocytes, are able to develop into either IL-10- and IFN--producing cells or into IL-5- and IL-13-producing cells, depending on the cytokines used in culture to raise the cells (15). At this moment, it is not known whether these two types of NK cells (dubbed NK1 and NK2, respectively) exist in vivo. However, it has been suggested that the strength of the early IFN- responses to RSV infection by NK cells determines, to a significant extent, the intensity of the involvement by CD8⁺ T cells in this process (14). Depletion of CD8⁺ cells, on the other hand, skews the immune response toward a Th2-driven eosinophil infiltration even in animals that are challenged with RSV after being sensitized against the F protein. As explained earlier, the F protein usually elicits a mononuclear, noneosinophilic lung response in mice (13). These data thus suggest that a complex chain of immune events occur during acute RSV infection in mice. Imbalances in any step of these events may be responsible for the skewing of the immune response toward the predominance of Th2-like cytokines and cells that is observed in children at risk for asthma. However, given the ineffectiveness of Th1-skewing treatment in preventing significant lung inflammation in RSV-infected mice (12), it is still unclear whether the Th2 bias is responsible for the acute LRI in these children or if it is otherwise only an early marker of an atopic predisposition.

	OUTCOME OF RSV LRIs IN CHILDREN

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

To further explore this issue, we have assessed the outcome of RSV LRIs occurring during the first 3 yr of life in a large cohort of children monitored from birth in Tucson, Arizona (16). We assessed lung function (before and after administration of a bronchodilator), total serum IgE levels, and prevalence of recurrent wheezing and of atopic sensitization at different ages between 6 and 13 yr in children with and without physician-ascertained RSV LRIs before age 3 yr (17). We found that, after adjusting for most known confounders, children with a history of RSV LRIs were at increased risk of recurrent wheezing at the age of 6 yr, but this risk markedly decreased with age and was not significant by the age of 13 yr. Moreover, children with RSV LRIs were not more likely to have high total serum IgE levels or to be sensitized to local aeroallergens at any age during childhood when compared with children with no history of LRIs in early life. Interestingly, forced expiratory volumes in 1 s (FEV₁) measured at age 11 yr were significantly diminished in children with a history of RSV LRIs, but these lung function deficits were readily reversible by use of inhaled albuterol. It is important to stress here that we had previously reported (in a study based on this same cohort) that diminished levels of lung function could be observed as early as during the first month of life in infants who would subsequently develop wheezing LRIs (18). These results thus suggested that RSV LRIs are associated with increased risk of subsequent wheezing during the preschool and early school years, but that this risk disappears by early adolescence. This questions the role of a Th2-skewed response as the link between RSV infection in early life and the subsequent development of recurrent wheezing during childhood. In fact, our data suggest that the main factor that determines both the development of airway obstruction during RSV infections in early life and persistent wheezing associated with these LRIs is either alterations in airway size, in the regulation of airway tone, or both. These are probably present before the LRI develops. Children who have RSV LRIs and who will go on to develop asthma later in life do have a skewed immune response to the virus, but the data do not support the contention that this skewed response is directly responsible for the acute RSV LRI in these children. These findings are also compatible with mouse-derived data (as described above) that question the role of a Th2-skewed response as a determinant of acute lung illness in this animal model.

	ALLERGIC SENSITIZATION AND THE DEVELOPMENT OF ASTHMA: THE ROLE OF TIMING

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

In the preceding discussion it has been stressed that early alterations in the immune response to environmental antigens, such as viruses, are important markers for a predisposition to the development of asthma. However, the most important factor associated with chronic asthma during the school years is sensitization to local aeroallergens (19). Perhaps one of the most interesting findings regarding the association between allergic sensitization and asthma is that this association is strongly age dependent. Although few children become sensitized to aeroallergens during the first 3 yr of life, the great majority of those who do become sensitized in this age period develop asthma-like symptoms later in life (20). The strength of the association between allergic sensitization and asthma decreases in relation to the age at which the child becomes sensitized. As a consequence, children who become sensitized to common aeorollergens after the age of 8-10 yr have a risk of developing asthma-like symptoms that is not much higher than that of children who do not become sensitized (21). It is thus not surprising that the factors that determine early sensitization to aeroallergens have become the subject of considerable research effort.

It now appears that early allergic sensitization is associated with a delay in the maturation of normal immune responses. Seminal studies by Prescott and coworkers (22) suggest that most children are exposed to perennial aeroallergens during fetal life, and that responses of PBMCs to aeroallergens at birth are skewed toward a Th2-like pattern. Although the exact cells responsible for this bias are not well known, these results suggest that an active process of attenuation of these Th2-like responses needs to take place postnatally for the child not to show IgE responses to aeroallergens during the first 2 yr of life.

The factors that determine the Th2 bias at birth and its subsequent attenuation have not been completely elucidated. Evidence suggests that antigen-presenting cells (APCs) such as dendritic cells are immature at birth, and are thus less likely to produce Th1-skewing cytokines (e.g., IL-12) at the time of antigen presentation. This may allow for the "default" Th2 bias to persist until the factors that determine APC maturation have had the chance to interact with the developing immune system (see below). Persistence of the Th2 bias in early life is believed to offer a window of opportunity for the selection of Th2-biased memory cells against those perennial aeroallergens that are most likely to be inhaled during this period of life. The consequent development of a chronic airway allergic reaction against these perennial aeroallergens at a time of crucial "physiologic" lung and airway remodeling may thus be the link between early allergic sensitization and the subsequent development of asthma. Persistent allergic inflammation is associated with the production of both growth factors and potent cytokines, and these substances can influence extracellular matrix deposition and, therefore, markedly disrupt physiologic airway remodeling. This may explain why sensitization that occurs after the critical period for lung growth (i.e., during the first 10 yr of life) is associated with less risk for the development of asthma.

Interestingly, reports from the longest ongoing follow-up of children with asthma enrolled during the early school years in Melbourne, Australia strongly support this hypothesis (23). When lung function of children with asthma was studied at different times between age 10 and 35 yr it was found that FEV₁ was significantly lower at the beginning of follow-up in subjects with severe asthma when compared with nonasthmatic subjects. However, lung function grew in parallel in these two groups during the follow-up period, and the deficits at the end of follow-up (age 35 yr) were similar to those observed at age 10 yr. This finding strongly suggested that most of the deficits in lung function present in subjects with severe asthma may already be established by the age of 10 yr, when lung and airway development is essentially complete. Our own work suggests that persistent wheezers (i.e., those children who wheeze during the first 3 yr of life and whose symptoms have not remitted by the age of 6 yr) have levels of lung function shortly after birth that are not significantly different from those of children who do not wheeze during the first 6 yr of life (7). It is tempting to speculate that alterations in lung function (and presumably in bronchial responsiveness) present in individuals with chronic asthma are the consequence of the chronic airway inflammation present in these subjects at a time of rapid physiologic airway remodeling, that is, infancy and early childhood.

	ENVIRONMENTAL FACTORS AND THE ATTENUATION OF Th2-LIKE IMMUNE RESPONSES

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

The scenario described above suggests that the environmental factors that determine the attenuation of Th2-like responses in early life are crucial for our understanding of the immune determinants of asthma. Moreover, understanding these environmental factors may also provide clues as to the causes of the marked increases in prevalence of childhood asthma, especially in developed countries. Our contention is that the marked increases in early sensitization to aeroallergens may explain, to a significant extent, the so-called asthma epidemic (24), and increases in early allergic sensitization are probably attributable to the elimination of "protective" exposures that attenuate Th2-like responses in early life. If, as postulated, a decisive role is played by the maturation of APCs, then exposure to microbial products in early life may be the decisive missing link.

It is now well established that the maturation of APCs is strongly modulated by exposure to microbial products (25). The most thoroughly studied among these products is lipopolysaccharide (LPS). LPS and LPS-like substances are present in the cell walls of many bacteria, including gram-positives, gram-negatives, and mycobacteria. These substances interact with CD14 and other cellular and extracellular elements of a complex receptor structure that is only beginning to be understood (26). These interactions allow for exquisitely sensitive nonadaptive reactions to noxious environmental stimuli, and these reactions may act as signaling mechanisms that modulate responses to a variety of other antigens in early life. There is evidence suggesting that CD14-mediated responses are immature in early life, and it has been postulated that these responses may mature after repeated exposures to LPS and related stimuli during early life (25). LPS is extremely abundant in nature, and is present both in the normal gut and in indoor dust and air (27). It is plausible to surmise that modern standards of cleanliness and hygiene may have radically decreased early life exposure to ingested and inhaled environmental LPS. Decreased exposure to LPS by inhalation, for example, may have delayed the maturation of the airway dendritic cell system, thus prolonging the period during which a Th2-like bias is predominant and allowing for early sensitization to aeroallergens.

This hypothesis is strongly supported by studies of the epidemiology of allergies in rural areas of Europe (E. Von Mutius, personal communication, 1999). It appears that prevalence of sensitization to local aeroallergens is significantly lower among children who live in the same areas but have no contact with such animals. It is plausible to surmise that living in contact with farm animals increases exposure to environmental LPS, although this speculation is in urgent need of empirical corroboration.

Further indirect support for the hypothesis that exposure to LPS and other bacterial products may play a role in the regulation of IgE comes from our own genetic studies. We (28) described a new, frequent polymorphism in the promoter region of the CD14 gene, which is located in chromosome 5q, an area of the human genome known to be linked to asthma and allergies (29, 30). When compared with the C allele, the T allelle of this polymorphism (especially in homozygous form) was associated with significantly higher circulating levels of CD14, with significantly lower total serum IgE levels, and with significantly higher number of positive skin tests among atopic subjects. These findings thus suggested that subjects who are perhaps genetically predisposed to having higher circulating CD14 levels may be more likely to respond to environmental factors such as LPS and thus to attenuate Th2-like responses to environmental stimuli.

	CONCLUSION

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

Advances in our understanding of the factors that determine immune responses to viruses and aeroallergens in early life may prove decisive in elucidating the complex interactions between genetic and environmental factors that are responsible for the inception of asthma. These advances may also shed light on what has induced the significant increases in asthma prevalence reported in developed countries during the last two decades.

	DISCUSSION

TOP INTRODUCTION EARLY VIRAL INFECTIONS AND... ANIMAL MODELS OF RSV... OUTCOME OF RSV LRIs... ALLERGIC SENSITIZATION AND THE... ENVIRONMENTAL FACTORS AND THE... CONCLUSION DISCUSSION REFERENCES

Weiss: Are all the persistent wheezers at age 13 atopic?

Martinez: Persistent wheeze is a predictive factor of hospitalization at age 6 and 11 as well as 13, irrespective of having atopy or not.

De Jongste: Is not FEV₁ too crude a measure of airways obstruction, particularly in children?

Martinez: Yes, it may or may not reflect changes in airway structure.

Von Mutius: Is there any specific characteristic of RSV infection as compared to other viral-induced wheezing episodes? In other words, what is the difference between RSV-induced wheeze disappearing over puberty and wheezy bronchitis defined as episodes of wheeze triggered by infection only?

Martinez: There is no specificity of RSV as compared to other viruses.

Platts-Mills: I agree that RSV is not the only virus that is relevant in early childhood; however, in our emergency room RSV is the predominant virus associated with acute episodes in children under 2 years old [Rakes and coworkers, AJRCCM 1999;159:785-790; Ed.].

Nevens: You proposed that RSV is related to relatively narrow airways. However, the question is whether progression to altered airway geometry is associated with inflammation induced by the RSV virus, in particular in patients at risk?

Martinez: The role of RSV is overemphasized, especially concerning the long-term effect. Remodeling is not an abnormal process, it occurs continually during growth. It is also dependent on other factors.

Postma: You drew our attention to a time window, in which a stimulus might have different effects on the development of atopy or asthma. Did you also examine the effect of the timing of the first RSV infection on the development of atopy or asthma, in particular the presence of wheeze and high/low IgE?

Martinez: Most wheeze onset is before 18 months of age. There is no difference with a narrowing of the age groups.

Björkstén: RSV contains a G protein which can activate IgE. Therefore, it would be plausible that RSV infection would correlate with the development of atopy.

Martinez: I do not deny the possibility. In some cases a severe reaction to RSV could be responsible for sensitization.

Sterk: I know that you purposely avoided mentioning the label "asthma." However, you ought to be congratulated for being the first to convincingly demonstrate the development of the key feature of asthma at age 11, namely variable airways obstruction, after RSV infection in early childhood. Regardless of an increase in serum IgE. What does this tell us about the relationship between atopy and asthma?

Martinez: That these are two syndromes of bronchial obstruction in childhood, one associated with atopy and the other associated with alterations in the regulation of airway tone.

Postma: Dr. Sterk referred to your observation that the children with RSV infections < 3 years, at age 11 had reversibility of airway obstruction, larger than those who did not have RSV infections in childhood, suggesting that this might reflect asthma, which is not necessarily so. Do these children also have hyperresponsiveness to methacholine, another phenotype of asthma?

Martinez: We did not assess the relation between methacholine responsiveness and RSV in early life in these children.

	Footnotes

Correspondence and requests for reprints should be addressed to F. D. Martinez, M.D., University of Arizona Respiratory Sciences Center, 1501 N. Campbell Avenue, Tucson, AZ 85724. E-mail: Fernando{at}resp-sci.arizona.edu

	References

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1. Yunginger, J., C. Reed, E. J. O'Connell, L. J. Melton, W. M. O'Fallon, and M. D. Silverstein. 1992. A community-based study of the epidemiology of asthma: incidence rates, 1964-1983. Am. Rev. Respir. Dis. 146: 888-894 [Medline].

2. Wright, A. L., L. M. Taussig, C. G. Ray, H. R. Harrison, and C. J. Holberg. 1989. The Tucson Children's Respiratory Study: II. Lower respiratory tract illness in the first year of life. Am. J. Epidemial 129: 1232-1246 .

3. Sears, M. R., G. P. Herbison, M. D. Holdaway, C. J. Hewitt, E. M. Flannery, and P. A. Silva. 1989. The relative risks of sensitivity to grass pollen, house dust mite and cat dander in the development of childhood asthma. Clin. Exp. Allergy 19: 419-424 [Medline].

4. Johnston, S. L., P. K. Pattemore, G. Sanderson, S. Smith, M. J. Campbell, L. K. Josephs, A. Cunningham, B. S. Robinson, S. H. Myint, M. E. Ward, D. A. Tyrrell, and S. T. Holgate. 1996. The relationship between upper respiratory infections and hospital admissions for asthma: a time-trend analysis. Am. J. Respir. Crit. Care Med. 154: 654-660 [Abstract].

5. Sigurs, N., R. Bjarnason, F. Sigurbergsson, B. Kjellman, and B. Björkstén. 1995. Asthma and immunoglobulin E antibodies after respiratory syncytial virus bronchiolitis: a prospective cohort study with matched controls. Pediatrics 95: 500-505 [Abstract/Free Full Text].

6. Pullan, C., and E. Hey. 1982. Wheezing, asthma, and pulmonary disfunction 10 years after infection with respiratory syncytial virus in infancy. Br. Med J. 5: 1665-1669 .

7. Martinez, F. D., A. L. Wright, L. M. Taussig, C. J. Holberg, M. Malonen, W. J. Morgan, and T. G. H. M. Associates. 1995. Asthma and wheezing in the first six years of life. N. Engl. J. Med. 332: 133-138 [Abstract/Free Full Text].

8. Martinez, F. D., D. A. Stern, A. L. Wright, L. M. Taussig, and M. Halonen. 1998. Differential immune responses to acute lower respiratory illness in early life and subsequent development of persistent wheezing and asthma. J. Allergy Clin. Immunol. 102: 915-920 [Medline].

9. Román, M., W. J. Calhoun, K. L. Hinton, L. F. Avendano, V. Simon, A. M. Escobar, A. Gaggero, and P. V. Diaz. 1997. Respiratory syncytial virus infection in infants is associated with predominant Th-2-like response. Am. J. Respir. Crit. Care Med. 156: 190-195 [Abstract/Free Full Text].

10. Openshaw, P. J.. 1995. Immunity and immunopathology to respiratory syncytial virus: the mouse model. Am. J. Respir. Crit. Care Med. 152 (Suppl.): S59-S62 .

11. Alwan, W. H., F. M. Record, and P. J. Openshaw. 1993. Phenotypic and functional characterization of T cell lines specific for individual respiratory syncytial virus proteins. J. Immunol. 150: 5211-5218 [Abstract].

12. Hussell, T., U. Khan, and P. Openshaw. 1997. IL-12 treatment attenuates T helper cell type 2 and B cell responses but does not improve vaccine-enhanced lung illness. J. Immunol. 159: 328-334 [Abstract].

13. Hussell, T., C. J. Baldwin, A. O'Garra, and P. J. Openshaw. 1997. CD8+ T cells control Th2-driven pathology during pulmonary respiratory syncytial virus infection. Eur. J. Immunol. 27: 3341-3349 [Medline].

14. Hussell, T., and P. J. M. Openshaw. 1998. Intracellular IFN- expression in natural killer cells precedes lung CD8+ T cell recruitment during respiratory syncytial virus infection. J. Gen. Virol. 79: 2593-2601 [Abstract].

15. Peritt, D., S. Robertson, G. Gri, L. Showe, M. Aste-Amezaga, and G. Trinchieri. 1998. Differentiation of human NK cells into NK1 and NK2 subsets. J. Immunol. 161: 5821-5824 [Abstract/Free Full Text].

16. Taussig, L. M., A. L. Wright, W. J. Morgan, H. R. Harrision, and C. G. Ray. 1989. The Tucson Children's Respiratory Study: I. Design and implementation of a prospective study of acute and chronic respiratory illness in children. Am J. Epidemiol. 129: 1219-1231 [Abstract/Free Full Text].

17. Stein, R. T., D. Sherrill, W. J. Morgan, C. J. Holberg, M. Halonen, L. M. Taussig, A. L. Wright, and F. D. Martinez. 1999. Respiratory syncytial virus in early life and risk of wheeze and allergy by age 13 years. Lancet 354: 541-545 [Medline].

18. Martinez, F. D., W. J. Morgan, A. L. Wright, C. J. Holberg, and L. M. Taussig. 1988. Diminished lung function as a predisposing factor for wheezing respiratory illness in infants. N. Engl. J. Med. 319: 1112-1117 [Abstract].

19. Halonen, M., D. A. Stern, A. L. Wright, L. M. Taussig, and F. D. Martinez. 1997. Alternaria as a major allergen for asthma in children raised in a desert environment. Am. J. Respir. Crit. Care Med. 155: 1356-1361 [Abstract].

20. Stern, D. A., I. C. Lohman, F. D. Martinez, A. L. Wright, L. M. Taussig, and M. Halonen. 1998. Presence of alternaria specific IgE during lower respiratory illnesses before age 3 (LRI<3) as a predictor of subsequent asthma (abstract). Am. J. Respir. Crit. Care Med. 157: A48 .

21. Peat, J. K., C. M. Salome, and A. J. Woolcock. 1990. Longitudinal changes in atopy during a 4-year period: relation to bronchial hyperresponsiveness and respiratory symptoms in a population sample of Australian schoolchildren. J. Allergy Clin. Immunol. 85: 65-74 [Medline].

22. Prescott, S. L., C. Macaubas, B. J. Holt, T. B. Smallacombe, R. Loh, P. D. Sly, and P. G. Holt. 1998. Transplacental priming of the human immune system to environmental allergens: universal skewing of initial T cell responses toward the Th2 cytokine profile. J. Immunol. 160: 4730-4737 [Abstract/Free Full Text].

23. Oswald, H., P. D. Phelan, A. Lanigan, M. Hibbert, J. B. Carlin, G. Bowes, and A. Olinsky. 1997. Childhood asthma and lung function in mid-adult life. Pediatr. Pulmonol. 23: 14-20 [Medline].

24. Woolcoock, A. J., and J. K. Peat. 1997. Evidence for the increase in asthma worldwide. Ciba Found. Symp. 206: 122-134 [Medline].

25. Holt, P. G., P. D. Sly, and B. Björkstén. 1997. Atopic versus infectious diseases in childhood: a question of balance? Pediatr. Allergy Immunol. 8: 53-58 [Medline].

26. Yang, R. B., M. R. Mark, A. Gray, A. Huang, M. H. Xie, M. Zhang, A. Goddard, W. I. Wood, A. L. Gurney, and P. J. Godowski. 1998. Toll-like receptor-2 mediates lipopolysaccharide-induced cellular signalling. Nature (London) 395: 284-288 [Medline].

27. Young, R. S., A. M. Jones, and P. J. Nicholls. 1998. Something in the air: endotoxins and glucans as environmental troublemakers. J. Pharm. Pharmacol. 50: 11-17 [Medline].

28. Baldini, M., M. Halonen, P. Holt, and F. D. Martinez. 1999. A polymorphism in the 5'-flanking region of the CD 14 gene is associated with circulating soluble CD14 levels and with total serum IgE. Am. J. Respir. Cell Mol. Biol. 20: 976-983 [Abstract/Free Full Text].

29. Postma, D. S., E. R. Bleecker, P. J. Amelung, K. J. Holroyd, J. Xu, C. I. Panhuysen, D. A. Meyers, and R. C. Levitt. 1995. Genetic susceptibility to asthma-bronchial hyperresponsiveness coinherited with a major gene for atopy. N. Engl. J. Med. 333: 894-900 [Abstract/Free Full Text].

30. Marsh, D. G., J. D. Neely, D. R. Breazeale, B. Ghosh, L. R. Freidhoff, E. Ehrlich-Kautzky, C. Schou, G. Krishnaswamy, and T. H. Beaty. 1994. Linkage analysis of IL4 and other chromosome 5q31.1 markers and total serum immunoglobulin E concentrations. Science 264: 1152-1156 [Abstract/Free Full Text].