Respiratory Syncytial Virus and Asthma
The Role of Monocytes

JAN L. L. KIMPEN

Department of Pediatrics, Wilhelmina Children's Hospital, Utrecht, The Netherlands

	INTRODUCTION

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

The monocytic cell line originates in the bone marrow as a common committed progenitor for both the granulocyte and monocyte/macrophage pathways (1). The earliest monocytic precursor in the marrow is believed to undergo two or three generations before the mature circulating monocyte is produced. Migration of circulating monocytes into tissues appears to be a random phenomenon in the absence of inflammation. Once in the tissues, monocytes undergo transformation into tissue macrophages, with morphologic and sometimes functional properties that are characteristic of the tissue in which they reside. Monocytes have also been shown to differentiate into dendritic cells (2), a class of leukocytes with specialized immunostimulatory capacities (3).

Monocytes, macrophages, and dendritic cells all play important roles in controlling the immune response in viral infections, not only by direct interaction with helper T cells and cytotoxic T cells but also by the production of cytokines (4). In addition, monocytes/macrophages can destroy some invading pathogens, and they have an important function as antigen-presenting cells (APCs). Monocytes/macrophages and, to a certain extent, dendritic cells appear to play a role in acute inflammation of respiratory syncytial virus (RSV) bronchiolitis and asthma exacerbations, and in the relationship between RSV and reactive airway disease (RAD) (5). On a clinical level, this means that these cells could have a role in the chronic inflammation that accompanies asthma, and probably also in recurrent wheezing after bronchiolitis.

	MONOCYTES AND ASTHMA

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

Some individuals with atopy have neither bronchial hyperresponsiveness nor asthma symptoms, whereas some normal individuals with neither atopy nor asthma symptoms have bronchial hyperresponsiveness when tested. In 1994, Poulter and associates (8) identified the links between T cell-mediated inflammation and bronchial hyperresponsiveness and proposed a hypothesis for the pathogenesis of asthma. In this hypothesis, the combination of immunologic and physiologic abnormalities may result in the promotion of disease. Poulter and colleagues argue that an integral factor in the prevention of these processes appears to be the regulation of T cell reactivity by a population of immunosuppressive lung macrophages. Only when bronchial inflammation and hyperresponsiveness occur in parallel, presumably because of failure of this macrophage-mediated T cell regulation, will symptoms of asthma develop.

	MONOCYTES AND RSV INFECTION

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

After RSV inoculation, the first cells the virus encounters are not T or B lymphocytes but respiratory epithelial cells, resident macrophages and dendritic cells (most probably in the lymph nodes), and monocytes in the airways. The immune response cascadeactivation of granulocytes (i.e., neutrophils, eosinophils, basophils) and the specific immune response, including activation of RSV-specific cytotoxic and helper T cells and production of RSV-specific immunoglobulinscan take place only after these cells are infected.

Monocytes and macrophages may have direct effects on the virus as the first line of defense in the airways in acute RSV infection. Monocytes and macrophages that interact with RSV also promote general and RSV-specific immunomodulatory effects, which may include a reduced immune response against other pathogens. A number of groups have studied the interactions between RSV and monocytes (9). These studies have shown that monocytes and macrophages internalize viral proteins and present these proteins on the cell membrane. Data from Panuska and associates (13) suggest that RSV can cause productive infection (i.e., the infected cell releases more viral particles than the number of particles infecting the cell) of alveolar macrophages. Human monocytes exposed to RSV express less intercellular adhesion molecule 1 (ICAM-1) and its ligand, lymphocyte function-associated antigen 1 (LFA-1), than do human monocytes exposed to influenza virus (15). These data suggest that exposure of monocytes to RSV fails to elicit or blocks the early events necessary for cellular collaboration, which contributes to early suppression of the clonal expansion of RSV-specific lymphocytes. Other studies have shown that RSV infection of monocytes/ macrophages increases the production of proinflammatory mediators, including tumor necrosis factor (TNF) (11, 16), interleukin 1 (IL-1) (15), IL-6 (11), IL-8 (11), IL-10 (10), platelet-activating factor (PAF) (14), and prostaglandin E₂ (16).

A general conclusion of all these studies is that RSV is able to induce monocyte activation, the overall result of which is immune inhibition with a reduction of lymphocyte transformation and, in some studies, a decreased expression of ICAM-1 and LFA-1 and a reduced interferon- (IFN-) response (17). The effect of RSV on the immune system, possibly through the interaction of the virus with monocytes, dampens the immune response, makes its reaction to infection less robust, and possibly creates an environment that is skewed to the helper T cell type 2 (Th2) side of the immunologic pathway.

All the above-cited studies were performed in monocytes and macrophages from adults. Takeuchi and coworkers (18) demonstrated that RSV interactions with monocytes may also occur in neonates. These investigators found that the level of transcription of interferon regulatory factor 1, IL-1-converting enzyme, and IL-1 in cord blood monocytes increased significantly 2 h after exposure to RSV as compared with uninfected monocytes. After 20 h of RSV exposure, quantities of IL-1 secreted from RSV-exposed monocytes remained moderately higher than from uninfected cells.

Bont and colleagues (19) compared the intensity of monocytic responses with the severity of disease in 30 children who required mechanical ventilation because of RSV bronchiolitis. At initiation of mechanical ventilation for respiratory failure, production of IL-10 and IL-12 was measured in 48-h peripheral blood mononuclear cell cultures stimulated with lipopolysaccharide and IFN-, thus inducing specifically monocytic responses. No correlation was found between monocyte IL-10 production and the duration of mechanical ventilation. However, a highly significant inverse correlation was found between the duration of mechanical ventilation and the production of IL-12 (p < 0.001) (Figure 1). Because IL-12 production is supposed to be an antiviral immune response, these data suggest that the lower the monocyte IL-12 response during clinical RSV infection, the more severe the disease may be. A corollary to this conclusion is that a low monocyte IL-12 response during an initial RSV infection adversely affects the clinical outcome of patients with severe RSV bronchiolitis.

View larger version (9K): [in this window] [in a new window]   Figure 1.   Monocyte interleukin 12 (IL-12) production during respiratory syncytial virus disease correlates inversely with days of mechanical ventilation. (Reprinted by permission from Bont L, Kavelaars A, Heijnen CJ, van Vught AJ, Kimpen JL. Monocyte interleukin-12 production is inversely related to duration of respiratory failure in respiratory syncytial virus bronchiolitis. J Infect Dis 2000;181:1772-1775.)

	MONOCYTES AND THE RELATIONSHIP BETWEEN RSV BRONCHIOLITIS AND RAD

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

To determine whether monocyte cytokine responses can be predictive of development of the well-documented recurrent wheezing that may follow RSV bronchiolitis, Bont and associates (20) measured monocytic IL-10 and IL-12 levels in whole blood cultures stimulated with lipopolysaccharide and IFN- from 50 children hospitalized with RSV bronchiolitis. Measurements were obtained during the acute phase of disease and 3 to 4 wk later. These responses were then analyzed in relation to the development of recurrent episodes of wheezing during a 1-yr follow-up period. IL-10 and IL-12 responses during the acute phase of RSV bronchiolitis were comparable to those of healthy control subjects. During the convalescent phase, IL-12 responses were not significantly different in patients and control subjects. In contrast, IL-10 responses were significantly increased in patients as compared with those in healthy control subjects (p < 0.001). At follow-up, 27 children (58%) had recurrent episodes of wheezing. The IL-10 levels measured during the convalescent phase were significantly higher in patients who developed recurrent wheezing during the year after RSV bronchiolitis than in patients without recurrent episodes of wheezing (p = 0.006) (Figure 2). There was also a linear correlation between the convalescent IL-10 levels and the number of wheezing episodes during the year after acute bronchiolitis (p = 0.004) (Figure 3). The mechanism by which monocyte IL-10 influences recurrent wheezing is not clear. Probably IL-10 influences other cells and activates cytokine networks, resulting in an "asthma-inducing" immune response on subsequent challenge with aspecific and allergic stimuli.

View larger version (12K): [in this window] [in a new window]   Figure 2.   Monocyte interleukin 10 (IL-10) production 3 to 4 wk after acute respiratory syncytial virus (RSV) bronchiolitis is associated with recurrent wheezing and physician-diagnosed asthma during the following year. Data represent individual values and means ± SEM. Data were analyzed after log transformation. (Reprinted by permission from Bont L, Heijnen CJ, Kavelaars A, van Aalderen WMC, Brus F, Draaisma JTM, Geelen SM, Kimpen JLL. Monocyte IL-10 production during respiratory syncytial virus bronchiolitis is associated with recurrent wheezing in a one-year follow-up study. Am J Respir Crit Care Med 2000;161: 1518-1523.)

View larger version (10K): [in this window] [in a new window]   Figure 3.   Ex vivo monocyte interleukin 10 (IL-10) production 3 to 4 wk after acute respiratory syncytial virus (RSV) bronchiolitis is associated with number of episodes of recurrent wheezing during the following year. Data represent individual values. Linear regression line is shown. (Reprinted by permission from Bont L, Heijnen CJ, Kavelaars A, van Aalderen WMC, Brus F, Draaisma JTM, Geelen SM, Kimpen JLL. Monocyte IL-10 production during respiratory syncytial virus bronchiolitis is associated with recurrent wheezing in a one-year follow-up study. Am J Respir Crit Care Med 2000;161:1518-1523.)

	CONCLUSIONS

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

There is now evidence that the direct interaction between T cells and RSV is not the only determinant of disease severity or subsequent recurrent wheezing. The interaction between RSV and the monocytic APC also plays a role, skewing this cell via the IL-12-type mechanism (i.e., Th1 response) toward a good antiviral response, with a faster recovery of disease, or toward an IL-10 type of response (i.e., Th2 response) and recurrent wheezing (Figure 4). The basis for this difference remains unclear but could be related to which viral proteins interact with the APC. It is clear that the RSV-APC interaction warrants more careful investigation, and that RSV-T cell and RSV-B cell interactions should not be considered the sole basis for recurrent wheezing.

View larger version (11K): [in this window] [in a new window]   Figure 4.   Summary of respiratory syncytial virus-monocyte/macrophage interactions. Abbreviations: APC = antigen-presenting cell; IFN- = interferon-; IL = interleukin; Th = helper T cell.

	DISCUSSION

TOP INTRODUCTION MONOCYTES AND ASTHMA MONOCYTES AND RSV INFECTION MONOCYTES AND THE RELATIONSHIP... CONCLUSIONS DISCUSSION REFERENCES

Piedimonte: Since IL-10 is an antiinflammatory cytokine, why is it linked with an increased risk of recurrent airway disease and bouts of inflammation?

Kimpen: We don't know. Perhaps the opposite is true, and inflammation is necessary for a favorable immune response against RSV, with no recurrent wheezing afterwards. We determined the levels of IL-10 and IL-12 produced by monocytes during acute RSV infection and correlated these levels with the seerity of disease and recurrent episodes of wheezing, but a lot remains to be learned about the pathophysiologic mechanisms behind these differences.

Rossi: Did you attempt to determine whether the results in acute patients were also present during the recovery period, to see if these are persistent findings?

Kimpen: No, we didn't look at that. Kapsenberg's group in Amsterdam (Kalinski P, Hilkens CM, Wierenga EA, Kapsenberg ML. T-cell priming by type-1 and type-2 polarized dendritic cells: the concept of a third signal. Immunol Today 1999;20:561-567) looked at antigen-presenting cellsmonocytes, dendritic cells, and macrophagesand reviewed the effect of microbial initiating factors on the immune response of these cells. So there is more than our very preliminary data suggesting that a certain interaction at a certain point in time during the acute infection, and maybe even in a particular organ systembe it the lungs, the peripheral lymph nodes, or the blooddetermines which way the antigen-presenting cell will steer the subsequen steps of the immune response.

Simoes: In the monkey model of RSV, we've shown that viral replication increases in more severely ill animals. Did you do RSV viral titers in the BAL fluid?

Kimpen: No, we didn't do quantitative viral analyses of the patient's BAL fluid.

Simoes: It may be important, because it is possible that the macrophages in the lung are all activated and being destroyed, while the circulating cells may very well be nonactivated.

	Footnotes

Correspondence and requests for reprints should be addressed to Jan L. L. Kimpen, M.D., Department of Pediatrics, Wilhelmina Children's Hospital, University Medical Center, KB 03.023.2, Lundlaan 6, 3584 EA Utrecht, The Netherlands. E-mail: j.kimpen{at}wkz.azu.nl

	References

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19. Bont L, Kavelaars A, Heijnen CJ, van Vught AJ, Kimpen JL. Monocyte interleukin-12 production is inversely related to duration of respiratory failure in respiratory syncytial virus bronchiolitis. J Infect Dis 2000; 181: 1772-1775 .

20. Bont L, Heijnen CJ, Kavelaars A, van Aalderen WMC, Brus F, Draaisma JTM, Geelen SM, Kimpen JLL. Monocyte IL-10 production during respiratory syncytial virus bronchiolitis is associated with recurrent wheezing in a one-year follow-up study. Am J Respir Crit Care Med 2000; 161: 1518-1523 .