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‘Local’ C3 opsonization of mycobacteria The study by Schorey et a1.14 also addresses the important issue of whether complement protein C3 secretion by the macrophage plays a role in C3 opsonization of bacteria. In this study, M. atGum incubated with macrophages from C3” mice in the presence of C2a is not taken up unless a source of C3 is provided, whereas bacteria associate with control macrophages under the same conditions. These experiments support the concept that mycobacteria can become C3-opsonized ‘locally’ in tissues by C3 secreted from the macrophage. The relative availability of proteases would dictate whether C3 b or C3 L-i predominates on the bacterium In this setting. The finding that C3 secreted by the macrophage can opsonize mycobacteria provides o.le possible explanation for the dis,covery that M. tuberculosis can interact with CRs in the absence of added serum’. Apart from issues related to complement protein ava lability, straindependent differences in the composition of M. tuberculosis outer ‘capsular’ polysaccharides have recently been shown to impact on C3 deposition to some extenP. These polysaccharides also mediate a nonopsonic interaction with CR3 expressed on Chinese hamster ovary (CHO) cells Is. The binding sites on CR3 to which the bacterium adheres (I domain, which recognizes C3bi vs. lectin sites) are predicted to impact on the cellular response (i.e. ligation of lectin sites leads to cellular activation such as the generation of an oxidative burst16). In this respect, the host cell response to mycobacterial adherence mav also be influenced by the relative’involvement of other macrophage receptors, such as the mannose receptor, which binds to M. tuberculosis lipoarabinomannanl or other CR3-associated receptors such as CD14 (Ref. 17). Summary Accumulated evidence to date confirms the importance of the CS-CR pathway in the phagocytosis of pathogenic mycobacteria. Detailed receptor-ligand studies for phagocytosis are creating the framework to test the hypothesis that the entry CopyrIght

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pathway for these bacteria influences the immediate host cell response and their intracellular fate. These types of study are particularly important for improving our understanding of the outcome of primary infection in humans, where the number of bacilli is presumed to be very low. References 1 Schlesinger, L.S.(1996) Curr. Top. Microbial. lmmunol. 215,71-96

2 Wright,S.D.andSilverstein, SC. (1983)

J. hp. Med. 158,2016-2023 3 Sutterwala, F.S. et al. (1997) J, Exp. Med. 185,1977-1985 4 Marth, T. and Kelsall, B.L. (1997) 1. Exp. Med. 185,1987-1995 5 Strunk, R.C., Eidlen, D.M. and Mason, R.J. (1988) J. C/in. Invest. 81, 1419-1426 6 McPhaden, A.R. and Whaley, K. (1993)

lmmunol. Res. 12,213-232 7 Ramanathan, V.D., Curtis, J. and Turk, J.L. (1980) Infect.lmmun. 29, 30-35 8 Rourke, F.J., Fan, S.S.and Wilder, MS. (1979) Infect. Immun. 23,160-167 9 Schlesinger, L.S. et al. (1990) J. Immunol. 144,2771-2780 10 Polotsky, V.Y. et al. (1997) ]. Infect. Dis. 175,1159-1168 11Hoppe, H.C. et al. (1997) Infect. Immun. 65,3896-3905 12 Matsushita, M. and Fujita, T. (1996) Res. lmmunol. 147,115-118 13 Schlesinger, L.S. and Horwi’itz, M.A. (1994) Infect. lmmwt. 62,280-239 14 Schorey, J.S., Carroll, M.C. and Brown, E.J. (1997) Science 277, 1091-1093 15 Cywes, C. et al. (1997) Infect. Immun. 65,4258-4266 16 Thornton, B.P. et al. (1996) J. Immunol. 156,1235-1246 17 Peterson, P.K. et al. ( 1995) Infect. lmmun. 63,1598-1602

Response from Brown and Schorey

pressedby rapid growers. Thus, this activity, unlike, for example, activation of the alternative pathway of r Schlesinger’s comments on complement, is associated with the the importance of C3-CR ability to invade macrophages suc(complement receptor) interaction cessfully in vivo. Although we have in infection of macrophages by not yet proved that C2a-dependent Mycobacterium tuberculosis help invasion is a true virulence factor, put into perspective our recent dem- this association is encouraging. onstration of a novel C2a-dependent We would like to take this oppormechanismby which virulent myco- tunity to point out that all the studbacteria can trigger this mechanism ies of macrophage-mycobacteria of uptake’. We agree that it is al- interaction in vitro fall short of demmost inconceivable that a pathogen onstrating relevance in vivo. The poso well adapted to intracellular sur- tential ability to answer somebasic vival in the macrophages of its host questions concerning pathogenesis would rely on a singlemechanismof in vivo has arisen with the developentry into thesecells. Undoubtedly, ment of mutant mouse strains devirulent mycobacteria have devel- ficient in potential pathways for oped multiple mechanismsby which macrophage invasion by mycobacthey can enter into host macro- teria. For example, mice deficient in phages to survive and occasionally both complement component C3 proliferate. The relative importance and in factor B, to abrogate alterof the various mechanismsthat Dr native pathway activation, existlT3. Schlesinger has reviewed will cer- Infection of these mice will help retainly depend on the specific bio- veal the significance of the alternachemicalcharacteristicsof the infect- tive pathway and of C3 activation ing strain, the site of infection, and in the establishment and course of whether macrophagesare examined mycobacterial infection. Mice with during initial infection, during the deficiencies of potentially relevant period of systemic dissemination of macrophage receptors, suchas CR1 diseaseor during reactivation dis- (Refs 4,s) and CR3 (Ref. 6), and ease.The significance of the C2a- with defects in macrophage matudependentmechanismof invasion is ration are also available’. Although that it seemsto be conserved among the mouse is not the natural host severalspeciesof intracellular patho- for M. tuberculosis, we believe that genic mycobacteria but is not ex- studies in thesemutant mice will be

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critical for defining the mechanisms involved in initiation and dissemination of mycobacterial infection. Although macrophages are the primary cells in which mycobacteria live during established infection, mycobacteria must cross an epithelial barrier either in the lung or in the gastrointestinal tract to initiate systemic Infection and also, presumably, for the host to develop a significant immune response. Basic questions regarding the establishment of infection, such as how mycobacteria cross this barrier, are extremely difficult to address in vitro. While it is generally assumed that alveolar macrophage infection is required for systemic dissemination of M. tu-

berculosis, evidence has been presented for mycobacterial transit across M cells8,9. Moreover, the gastrointestinal tract, which, unlike the lung, has no intraluminal macrophages, is a frequent site of infection for both Mycobacterium bovis and Mycobacterium avium. Thus, the relevant site for initial infection of macrophages remains unknown.

References

1 Schorey, J.S.,Carroll,M.C. and 2 3 4 5

Eric Brown and Jeff Schorey Divn of Infectious Diseases, Dept of Medicine, Dept of Molecular Microbiology, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, MO 63110-1093, USA

6 7 8 9

Brown, E.J. (1997) Science 277, 1091-1093 Wessels, M.R. et al. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 11490-11494 Matsumoto, M. et al. (1997) hoc. Nat/. Acad. Sci. U. S. A. 94,8720-8725 Ahearn, J.M. et al. (1996) Immtmif? 4, 251-262 Molina, H. et al. (1996) hoc. Nat/. Acad. Sci. U. S. A. 93,3357-3361 Coxon, A. et al. (1996) Immunity 5. 653-666 Wiktor-Jedrzejczak, W. and Gordon. S. (1996) Physiol. Rev. 76. 927-947 Momotani, E. et al. (1988) Vet. Patho/. 25,131-137 Fujimura, Y. (1986) Gastroenterol. jpn. 21,325-335

Letter Theory, data and experiments transmission atthew Keeling’s recent review’ of the persistence of measles clearly and elegantly highlights the importance of spatial and temporal heterogeneity in the dynamics of disease. One of the main challenges in modelling measles remains the capture of’ patterns of fade-out and re-occurrence in cities of different sizes’. It is a tribute to Bartlett’s insight half a century ago that the idea of the critical community size (CCS), above which measles persists, remains a major element in this work.

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Usinga combinationof ecological theory, mathematical modelling and data analysis, Keeling neatly shows that simple deterministic models, which are widely used in epidemiology, cannot capture the dynamics of measles without incorporating greater biological and social realities that impose temporal and spatial heterogeneities on the transmission of disease. Yet some competing hypotheses remain unresolved about the relative importanceof temporal heterogeneities resulting from seasonal mixing, age structure in the susceptible populations, and spatial heterogeneities involving transmission between loosely coupled subpopulations or scaling up from individual families to population behaviour. Keeling favours more accurate descriptions of latent and infectious periods as the simplest Copyright

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means of explaining the CCS, but the precise role of each heterogeneity has yet to be resolved. What lessons do these results hold for infectiousdiseaseepidemiologyin general? Although extensive prevalence data are available, the complexity, diversity and size of human social networks makes modelling transmission difficult: as Keeling points out, estimating the degree of coupling between subpopulations is very hard. But many mammal populations have small, distinct and homogenous social units with relatively well-defined coupling between subpopulations, which makes them amenable to metapopulation approaches. The failure of a model to achieve a biologically plausible CCS for a disease may impugn the underlying assumptions (particularly the ‘susceptible, exposed, infectious, recovered’ paradigm), but some natural metapopulations are simply not large enough for disease to persist. Recent work has shown that the decline and fade-out of the phocine distemper virus epidemic that spread through North Sea harbour seals in the 1980s can be explained because the CCS calculated for a stochastic metapopulation model with mixing within and between neighbouring haulouts was larger than the global populations of harbour seals’.

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The CCS is a concept specific to infections (with short generation times but no persistence within individuals) that rely on a sustained supply of susceptible individuals, but the approaches towards analysing

heterogeneous mixing and seasonal forcing, which lead to it, are much more general. Many important plant diseases (caused by nematodes, fungi and certain viruses) are transmitted through dispersal of inoculum that is localized within fields or smaller patches of susceptible but static hosts. Local spatial correlations dominate these epidemics, with transmission

betweensubpopulationstendingto be restricted to neighbouring subpopulations. New likelihood-based statistical techniques can exploit this structure to estimate local transmission“. The damping-off fungus, Rhizoctonia solani, offers a tractable experimental system of rapid (14 d) epidemics in replicated microcosms of radish seedlings’. Even here, in tightly controlled microcosms, dynamically generated variability arises because of nonlinear amplificat:ion of small initial differenceG, and variability within and between replicate epidemics can be explained and reproduced in relation to temporal and spatial heterogeneitiesh. Mapping of disease, interruption of disease transients, seasonal forcing, and pulsing of inputs or removals are .ilI experimentally tractable’. Future progress in unravelling the epidemiological mechanisms of heterogeneous transmission is likely 842X/98/$1Y.O0

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