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Infection and Immunity, May 2000, p. 3064-3065, Vol. 68, No. 5
0019-9567/00/$04.00+0
LETTERS TO THE EDITOR
Which Routes Do Plasmodium Sporozoites Use for
Successful Infections of Vertebrates?
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LETTER |
In mice, sporozoites of Plasmodium berghei delivered by
mosquito bites are significantly more infectious than those transmitted by intravenous (i.v.) inoculation, as shown by Vaughan et al. (3). Using the chicken malaria P. gallinaceum in
its natural host we obtained similar results. Sporozoites delivered by
bites of two to five Aedes fluviatilis mosquitoes infected
100% of 1-week old chicks after prepatent periods (PPP) of 4 to 7 days, resulting in mortality of all birds. Sporozoites injected by
syringe also cause malaria, although with lower parasitemia, longer PPP
(~11 days), and lower mortality (40% to 75%). Unlike P. berghei and all other sporozoites that infect mammals (which
develop in the hepatocytes), P. gallinaceum sporozoites
initially invade and develop within skin macrophages at the site of
injection, rather than in other tissues.
The route by which sporozoites reach the hepatocytes is still debatable
although their suggested transport via the lymphoid system
(3) could well be through macrophages and/or Küppfer cells. The fact that P. berghei sporozoites enter and leave
macrophages without being destroyed and that all attempts to cultivate
the mammalian plasmodium sporozoites in hepatocytes have resulted in an
extremely low percentage of infections supports this hypothesis. In
addition, opsonized P. berghei sporozoites phagocytized by macrophages or Küpffer cells are destroyed.
In the presence of stage-specific monoclonal antibodies (MAb of 
3
µg), sporozoite invasion and/or development in macrophages is totally
abrogated (2), indicating that in vitro, the primary exoerythrocytic forms of P. gallinaceum developing inside
macrophages are susceptible to being killed by antibodies. A direct
correlation occurs between the protective effect of MAb in vitro and in
vivo. Thus, in vitro suspensions of sporozoites plus MAb injected i.v. did not cause infection when we used the active MAb. All control chicks
receiving sporozoites with medium or specific MAb with no activity in
vitro had patent malaria and high parasitemia (2).
In the mouse model, high doses of passively transferred specific MAb
antisporozoites inactivated sporozoites given i.v. but not through
mosquito bites (3). This result strongly suggests the
presence of protective mechanisms other than the blocking of sporozoite
invasion into the host cell by MAb. Furthermore, in mice challenged
with mosquito bites, protection hardly occurred, despite MAb transfer.
We propose that macrophage killing of the opsonized sporozoites did not
occur because the parasites were taken up by skin macrophages in the
presence of low immunoglobulin G levels not sufficient to opsonize the parasites.
Finally, since high homology between DNA sequences of the
circumsporozoite genes has been described for P. falciparum
and P. gallinaceum (1) supporting a close
phylogenetic relationship between these two species, it is quite
possible that other similarities between the life cycles of avian and
mammalian malaria parasites do exist. However, the role of macrophages
in sporozoite transport and/or in antibody-mediated destruction of
P. falciparum sporozoites and other mammalian malaria
parasites is highly relevant to vaccine development and deserves
further study, since antibodies are the key to antisporozoite protection.
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REFERENCES |
| 1.
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McCutchan, T. F.,
J. C. Kissinger,
M. G. Touray,
M. J. Rogers,
J. Li,
M. Sullivan,
E. M. Braga,
A. U. Krettli, and L. H. Miller.
1996.
Comparison of circumsporozoite proteins from avian and mammalian malarias: biological and phylogenetic implications.
Proc. Natl. Acad. Sci. USA
93:11889-11894[Abstract/Free Full Text].
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Ramirez, A. D.,
E. M. Rocha, and A. U. Krettli.
1995.
Antisporozoite antibodies with protective and non-protective activities: in vitro and in vivo correlation using Plasmodium gallinaceum, an avian model.
J. Eukaryot. Microbiol.
42:705-708[Medline].
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| 3.
|
Vaughan, J. A.,
L. F. Scheller,
R. A. Wirtz, and A. F. Azad.
1999.
Infectivity of Plasmodium berghei sporozoites delivered by intravenous inoculation versus mosquito bite: implications for sporozoite vaccine trials.
Infect. Immun.
67:4285-4289[Abstract/Free Full Text].
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Antoniana U. Krettli
Luciana A. B. Dantas
Centro
de Pesquisas René Rachou/FIOCRUZ
Av. Augusto de Lima,
1715
30190-002 Belo Horizonte, MG
Brazil
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AUTHOR'S REPLY |
The question of how mosquito-transmitted sporozoites reach the liver is
longstanding and unresolved. We now know that most sporozoites are not
injected directly into the bloodstream, as is commonly depicted in life
cycle diagrams. Since the early works of Boyd and Kitchen
(1), evidence has steadily accumulated to indicate that
during mosquito probing, most transmitted sporozoites are deposited as
clumps within the skin and that there is a substantial delay in the
movement of mosquito-transmitted sporozoites away from the site of the
mosquito bite (4, 6). There are two possible routes that
mosquito-transmitted sporozoites can take to move away from the bite
site
entry into efferent capillaries directly or, more plausibly, via
lymphatic drainage (3).
If mosquito-transmitted sporozoites enter the circulation via the
lymphatics, then it is all the more remarkable that they are so
efficient in reaching the liver. The lymphatic route is circuitous and
seemingly fraught with danger. In order to reach the liver via the
lymphatics, mosquito-transmitted sporozoites must pass through lymph
nodes to reach the thoracic or right lymphatic ducts which then empty
into brachiocephalic veins and into the superior vena cava. Sporozoites
would then pass through the right atrium/ventricle and into the
pulmonary circulation, including a passage through the alveolar
capillary plexus. The left atrium/ventricle would propel the
sporozoites through the aortic arch and descending aorta. If
sporozoites were fortunate enough to enter the celiac trunk on their
first pass through the systemic circulation, they may be sent directly
into the liver via the common hepatic artery or, more likely, into the
other arterial branches of the celiac trunk to the stomach, pancreas,
or spleen. A bit further down the aorta, sporozoites might be sent into
the superior mesenteric artery. In either case, sporozoites would have
to pass through capillary beds of the lower digestive system before
entering the hepatic portal system and arriving at the relative calm of
the sluggish circulation within the liver sinusoids.
For mammalian plasmodia, the traditional view holds that sporozoites
travel to the liver extracellularly. Drs. Krettli and Dantas offer an
alternative scenarioone inspired by their work with avian plasmodia
and to which I refer to informally as the "taxicab hypothesis." In
this scenario, mosquito-transmitted sporozoites quickly invade
macrophages (or some other leukocyte type) in the skin and are then
carried inside of host leukocytes with the draining lymph, away from
the bite site, through the perilous lymph nodes, and on to the liver.
It has been demonstrated that sporozoites are fully capable of
"actively and aggressively" moving into and out of macrophages
(8). Indeed, the bioactive substances in mosquito saliva may
potentiate host edema and leukocyte infiltration to the site of
sporozoite deposition (2, 5). As Drs. Krettli and Dantas
suggest, the taxicab hypothesis may explain why many mosquito-transmitted sporozoites are able to elude the host protective effects of passively administered anticircumsporozoite monoclonal antibodies, whereas many intravenously inoculated sporozoites (i.e.,
"naked" sporozoites) are not (9).
The mechanism(s) by which mosquito-transmitted sporozoites complete
their journey to the liver remains unknown. But if a sporozoite vaccine
is to succeed, the biology of this journey needs to be elucidated. In
his classic work on sporozoite transmission (7), Vanderberg
noted that "...until it becomes possible to label sporozoites and
track them...there seems no way to assess the total numbers actually
inoculated by mosquitoes" or, in this case, to determine how
mosquito-transmitted sporozoites reach the liver. Recent success in
producing a stably transformed line of P. berghei in which sporozoites express green fluorescent protein (Kenneth Vernick, personal communication) may prove useful in monitoring the progress of
mosquito-transmitted sporozoites as they move from the skin to the liver.
| |
REFERENCES |
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|
Boyd, M. F., and S. F. Kitchen.
1939.
The demonstration of sporozoites in human tissues.
Am. J. Trop. Med.
9:27-31.
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Goldman, L.,
E. Rockwell, and D. F. Richfield.
1952.
III. Histopathological studies on cutaneous reactions to the bites of various arthropods.
Am. J. Trop. Med. Hyg.
1:514-525.
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Griffiths, R. B., and R. H. Gordon.
1952.
An apparatus which enables the process of feeding by mosquitoes to be observed in the tissues of a live rodent; together with an account of the ejection of saliva and its significance in malaria.
Ann. Trop. Med. Parasitol.
46:311-319[Medline].
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Ponnudurai, T.,
A. H. Lensen,
G. J. van Gemert,
M. G. Bolmer, and J. H. Meuwissen.
1991.
Feeding behaviour and sporozoite ejection by infected Anopheles stephensi.
Trans. R. Soc. Trop. Med. Hyg.
85:175-180[CrossRef][Medline].
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Rockwell, E. M., and P. Johnson.
1952.
The insect bit reaction. II. Evaluation of the allergic reaction.
J. Investig. Dermatol.
19:137-155[Medline].
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Sidjanski, S., and J. P. Vanderberg.
1997.
Delayed migration of Plasmodium sporozoites from the mosquito bite site to the blood.
Am. J. Trop. Med. Hyg.
57:426-429.
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| 7.
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Vanderberg, J. P.
1977.
Plasmodium berghei: quantitation of sporozoites injected by mosquitoes feeding a rodent host.
Exp. Parasitol.
42:169-181[CrossRef][Medline].
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| 8.
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Vanderberg, J. P., and M. J. Stewart.
1990.
Plasmodium sporozoite-host cell interactions during sporozoite invasion.
Bull. W. H. O.
68(Suppl.):74-79.
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| 9.
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Vaughan, J. A.,
L. F. Scheller,
R. A. Wirtz, and A. F. Azad.
1999.
Infectivity of Plasmodium berghei sporozoites delivered by intravenous inoculation versus mosquito bite: implications for sporozoite vaccine trials.
Infect. Immun.
67:4285-4289.
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| | | | |
Jefferson A. Vaughan
Department of Biology
University of North Dakota
Grand Forks, North Dakota 58202
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Infection and Immunity, May 2000, p. 3064-3065, Vol. 68, No. 5
0019-9567/00/$04.00+0
