Download Theoretical Article The importance of T cell homing and the

Immunology and Cell Biology (1992) 70: 405-410
Theoretical Article
The importance of T cell homing and the spleen in
reaching a balance between malaria immunity and
immunopathology: The moulding of immunity by early
exposure to cross-reactive organisms
MICHAEL F. GOOD and JEFF CURRIER
Queensland Institute ofMedical Research, Brisbane, Queensland, Australia
Summary It takes a number of years to develop clinical immunity to malaria and malaria pathology
is also most evident a number of years after birth. T cells are known to play an important role in
defence from malaria parasites but may also contribute to the disease symptoms associated with
malaria. T cells which react against malaria parasites have arisen through stimulation with organisms
which cross-react with malaria or through exposure to the malaria parasites themselves and express a
memory phenotype (CD45Ro *, CD45Ra " , CD4 "^). T clones which have arisen through exposure
to cross-reactive organisms may he expected to home to the tissues where initial exposure occurred as
determined by tissue-specific adhesion molecules on the lymphocyte surface. Such tissues may not be
appropriate to parasite killing and localization of T cells in such sites may contribute to the
immunopathology of malaria. The sharp increase in immunity and decline in pathology observed in
later childhood in malaria endemic areas may result from an increase in the number of T cells induced
by the parasite itself (as opposed to cross-reactive organisms). Such T cells may not have a preferential
trafficking to other organs and may be more likely to circulate through the spleen. Splenic changes
may also allow more malaria-specific T cells to concentrate in the spleen and may facilitate
interactions between T cells, monocytes, neutrophils and parasites resulting in parasite death.
Whereas cytokines secreted by parasite-reactive T cells in all locations may contribute to cerebral
malaria and other forms of pathology, cytokines in the spleen at least, should directly contribute to
parasite death.
Key words: cytokines, lymphocytes, malaria, monocytes, neutrophils, spleen T cells.
T cells and anti-parasite immunity
Exposure of T cells from either malariaexposed or immune individuals to malaria
parasites induces significant responses. An unexpected finding, however, has been that T
cells from non-exposed individuals respond in
vitro as well as T cells from exposed individuals. Such T cells proliferate following
incubation with parasites (perhaps up to 1 % of
peripheral T cells) and they secrete numerous
cytokines, including those implicated in parasiticidal activity (e.g. y-interferon)'. T cell
receptor (TCRJ-a/P cells respond in a major
histocompatibility complex (MHC)-restricted
manner,^ but TCR-7/0 T cells may also be
activated.-^ The cells that respond have been
shown to express a memory phenotype
(CD45Ro),^''* and directly or indirectly can
cause parasite death in vitro,^ in keeping with
data collected from experimental animals in
vivo showing that passively transferred T cell
Correspondence: M. F. Good, Queensland Institute of Medical Research, 300 Herston Road, Brisbane,
Qld 4029, Australia.
Accepted for publication 27 August 1992.
406
M. K Good and]. Currier
lines and clones derived from malaria-immune
animals can protect naive animals from malaria challenge.^ Evidence suggests that
malaria-specific T cells from non-exposed individuals have arisen as a result of crossreactive stimulation by other organisms, as
demonstrated by expression of CD45Ro^'^
and by the patterns of cross-reactivity of
malaria-specific clones (Fig. l).'^ Where differences have been observed between the T
cell responses of exposed and non-exposed
individuals in vitro, they have involved differences in the fine repertoire of specific T cells/
Epitopes have been defined which are recognized by T cells from only malaria-exposed
individuals and not from non-exposed individuals. We believe that these epitopes represent
those that are not found in other organisms to
which individuals are commonly exposed. Although it may seem obvious, this is the best
(perhaps only) evidence that the parasite itself
can induce and activate human precursor T
cells. Thus, we believe that malaria-specific
memory T cells can be divided into those
responding to cross-reactive epitopes present
in other organisms, and those responding to
epitopes uniquely present in malaria sequences. However, the relative efficacies of
malaria-specific T cells induced by the parasite
and those induced by cross reactive organisms,
in terms of protection, is unknown. T cell
homing patterns may be expected to be different between the two cell populations, and this
may have a major effect on the expression of
immunity and immunopathology.
Memory cells have tissue tropism, mediated
by selectins, integrins and other adhesion
molecules.^ Naive cells, as they leave the
thymus, express surface markers (some of
which have been defined) and direct homing
to lymph nodes through the post-capillary
high endothelial venules (HEV). This is mediated by MEL14/LAMl(Leu8) on murine and
human T cells. Following activation in the
lymph node by antigen presenting cells (APC)
from draining tissue, cells leave by the efferent
IB 19 20 21 22 23 2* 25 26 27 2a 29 30 31 32 33 3* J5 36
Antigen
Fig. 1. Cross-reactivity between a typical malaria-induced T cell clone (JC32) from a non-exposed
individual and common environmental organisms, from Currier et al.,^ with permission. Antigens tested
were as follows: (1) Plasmodium falciparum parasitized RBC; (2) Adenovirus types 3,4,6,7; (3) Cytomesahuirus;
(4) Epstein-Barr virus; (5) HSV (1 +2); (6) Influenza A2 virus; (7) Influenza B vims; (8) Measles vims;
(9) Mumps virus; (10) Parainfluenza vims 1; (11) Parainfluenza vims 2; (12) Parainfluenza vims 3;
(13) Poiiouirus types 1,2,3; (14) Rotavirus; (15) VZV; (16) Mycoptasma pneumoniae; (17) Toxoplasma gpndii;
(18) Escherkhia coli:, (19) Staphylococcus aureus; (20) Streptococcus pyo^enes; (21) Pseudomonas aeru^inosa;
(22) Alternaria mix; (23) Aspergillus mix; (24) Boirytis cinerea; (25) Candida alhicans; (26) Epidermoplryton;
(27) Fusarium vasinfectum; (28) Microsporum mix; (29) Mucor racemosus; (30) Rhizopus nigricans;
(31) Trichophyton mix; (32) Penicillium mix; (33) Bordetella pertussis', (34) Trichomonas va^inalis; (35) PPD;
(36) Tetanus toxoid.
T cells, the spleen and malaria immunity
407
lymph. These cells may have down-regulated
MEL14 but express other markers including
certain activation markers such as C D 4 5 R Q (in
humans). These cells also acquire tissue tropic
markers, such as cutaneous lymphocyte antigen (CLA) and CD44, which direct cells to
'addressins' on specific endothelium (e.g.
ELAM-1 on skin vascular endothelium). Thus,
cells can quickly home to the appropriate
tissue most likely to be infected with a particular organism. Thus, site of exposure is
'remembered'. A study by Brake el al. in 1988
showed that one out of 10 CD4 T cell clones
specific for blood stage P. chabaudi (a rodent
malaria) was able to transfer immunity in
vivo.^ Unless the cytokine profiles and functions of the various clones were critically
different, this result was difficult to explain
since the effector function of blood stage
malaria-specific T cells is thought to be nonspecific. A recent study by Rodrigues et al.,
however, with CD8 cytotoxic T lymphocyte
(CTL) clones specific for the circumsporozoite
protein (CSP) of P, berghei, showed that different clones which displayed identical fine antigen specificity, cytotoxic capacity in vitro, and
similar y-interferon secretion profiles, differed
in their ability to eliminate sporozoiteinfected hepatocytes in inuo.^ They were able
to show that expression of the homing molecule CD44 correlated with in vivo activity.
Their data suggested that CD44, which has
been shown to mediate transendothelial
migration and activate LFA-1-mediated aggregation of T cells, was required to direct the
protective cells between cndothelial cells to
liver target cells.
gradually increase from birth until age 4-, and
then drop sharply.'^ Absolute numbers of
deaths from malaria peak between ages 1- and
4- and then quickly decline. Cerebral malaria
peaks at age 4^." Electron micrographic studies have shown that human red cells parasitized with P. falciparum are able to adhere to
vascular endothelium in the brain (cytoadhercnce).
Parasitized erythrocytes also
clump with other red cells and with white
blood cells and these events may all contribute
to cerebral haemostasis and the symptoms of
cerebral malaria. Many people who recover
from cerebral malaria have, however, no residual effects, suggesting that factors other than
just cyto-adherencc may also be responsible. In
animal models of cerebral malaria, removal of
CD4 T cells has been shown to prevent
cerebral malaria, and antibodies against tumour necrosis factor (TNF)-a have been
shown to have a similar effect.^"^'^* That such
factors might also be of importance in human
cerebral malaria was suggested by studies
showing that serum TNF levels were significantly higher in malaria patients who developed cerebral symptoms,'^ and that T cell
lymphokines were able to up-reguiate adhesion molecules in the brain for P. jalciparum
parasitized red blood cells (p-RBC), for example, ICAM-1. Cytokines themselves may trigger a nitric oxide response in the vascular
endothelium and surrounding brain tissue
causing unco-ordinated neuronal activation
and significant pathology,"* and TNF-a as
well as lymphotoxin appear to contribute to
the hypoglycaemia associated with severe
malaria.'^
In the above study, T cells were induced by
sporozoite exposure. However, if malariaspecific cells have arisen through exposure to
other organisms in a particular site, then these
malaria-specific cells may return to such an
environment. In the wrong environment,
such cells may not contribute to parasite death,
but to immunopathology (e.g. cerebral malaria).
Thus paradoxically, on one hand, T cells
may contribute significantly to immunity, but
on the other hand they may cause serious
pathology. Since T cells responsive to malaria
may be present in significant tiumbers prior to
any contact with malaria, exposure of naive
individuals to parasite can then follow either
of two courses: either their pre-existing
malaria-specific T cells can secrete cytokines
contributing to serious pathology; or, more
commonly, after a period of less severe illness,
parasites are controlled (as a result of these
specific T cells, but involving other factors as
well) and the patient recovers. Re-exposure of
previously exposed individuals can also follow
T cells and disease
In west Africa, careful studies have shown that
the percentages of death as a result of malaria
408
M. F. Good and]. Currier
either of these courses, but with more exposure parasite control is a more common
finding. What tips the scales in either direction and controls the outcome of encounter
with parasite? With more exposure, more
malaria-specific T cells will have been induced
by the parasite in the spleen. Such cells may
not have a preferential tissue homing and may
circulate through the spleen more frequently.
Studies in humans and animals show that the
spleen plays a critical role in immunity to
malaria.
The spleen and malaria
Important literature from the early years of
this century clearly showed that the spleens of
humans increased in size with exposure to
malaria.^^ The phenomenon is so reliable that
'splenic rates' (the frequency of palpably enlarged spleens in a conununity) became an
index of malaria incidence, and splenomegaly
was viewed merely as a sign of disease. The
early data showed that in endemic settings,
spleen rates increased from birth until approximately age 4-5 years and then decreased
slightly. In areas of higher endemicity, jpleen
rates peaked a little earlier than in areas of
lower endemicity. That splenic changes may
be more than just a sign of disease was
suggested again by some early studies. Case
reports of malaria recurring following splenectomy but many years after a previous attack
have been published.'^'^" A number of experiments in monkeys, rats and mice supported an
important role for the spleen by showing that
resistance to, and immunity from malaria were
usually dependent on an intact spleen, although there are certain models where the
spleen appears to be less important. In P.
falciparum, the spleen also plays a crucial role
in keeping mature forms, the forms known to
adhere to vascular endothelium in the brain,
out of general circulation. Splenomegaly in
response to malaria involves an increase in cell
numbers, but, in mice at least, also appears to
involve a significant change in the architecture of the spleen, in particular the formation
of a blood-spleen barrier which harbours developing erythroblasts and protects them from
the parasite.^' The changes in architecture
also appear to facilitate interaction between
pRBC and T cells and monocytes/neutrophils. The evidence for this comes from
murine studies that show that: (i) splenectomy
of immune mice completely abrogates immunity even if the immune spleen cells arc transferred back into the splenectomized mice; and
(ii) while depletion of CD4 T cells from P.
fiMcfeer-immune mice is able to completely
abrogate immunity, immune CD4 T cells can
only transfer immunity into previously immune mice (which were subsequently CD4depleted) and not into naive mice.^^
So there is considerable evidence that the
spleen is more than just a marker of malaria
exposure, and that splenomegaly and splenic
changes probably occur as a critical event in
the development of immunity to malaria. This
is not to say that splenic changes are the only
important elements in immunity.
Immunity versus disease:
The Hypothesis
It is suggested here that the origins of the T
cells responsible for disease and immunity
differ. T cell clones expanded in vivo by
cross-reactive organisms will 'home' to the
tissues where the antigen was first encountered, as mediated by selectins, integrins and
other adhesion molecules on the surface of the
memory T cell, whereas T cell clones expanded in vivo by malaria parasites will not
nave a preferential tissue location and will
more frequently recirculate through the
spleen where their anti-parasite effect is maximal. The cells may lack tissue specificity
because they have not been processed and
activated by the draining nodes of the tissues
where imprinting of the homing pattern appears to occur;^ rather they first encounter
malaria parasites in the blood stream and
spleen. The T cells that do home to the
tissues, if malaria cross-reactive, will be relatively ineffective parasite killers, as demonstrated by the various observations that
splenectomy can abrogate immunity even if
the T cells are re-infused. However, T cell
activation in tissue locations will result in
TNF-a and lymphotoxin production which
can directly contribute to pathology either
T cells, the spleen and malaria immunity
locally or at a distance, as explained above.
Whereas cytokine production will also occur
in the spleen, at least in that location T cells
and their factors can effectively participate in
parasite killing. Age-related and parasiteinduced changes in the spleen increase the
efficiency of the spleen in mediating parasite
death.
The decline in mortality from malaria in
later childhood may be the result of splenic
changes and an accumulation of T cells which
have been induced by parasite, as opposed to
cross-reactive organisms. Although changes
other than simple splenomegaly may be important, the increased incidence of spleen rates
in children as a factor of age may reflect other
related changes.
Much attention has been paid to the toxic
side effects of phosphoUpids in malaria. These
stimulate macrophages directly to secrete
TNF.^"* These effects may be important, but
appear to be separate from the action of T
cells, stimulated by protein to secrete
7-interferon, to stimulate monocytes. The
magnitude of the in vitro response of T cells
to even low numbers of parasites (1-10/p-L
blood) strongly suggests that this mechanism
is of major importance.^ While antibodies
against phospholipids may block the pathology in that system, they would not be expected to block the pathology mediated
through T cells.
It has been previously suggested that the
immune response to a malaria vaccine may
trigger immunopathology.^'' The hypothesis
outlined here would not contradict that claim.
Furthermore, we would argue that the mode
and site of administration of a malaria vaccine
would be critical to establish the appropriate
setting for the pattern of malaria-specific T
cell homing. They also must be able to facilitate appropriate changes in the spleen that
permit effective parasite killing in that organ.
Live vaccines that target the spleen may be
ideal since any malaria-specific cells induced
by such a vaccine may not preferentially traffic
to other tissues. The consequent reduction in
parasites circulating to other organs and the
likely large increase in specific T cells in the
spleen will reduce pathology in other organs.
409
Acknowledgements
We thank R. Carter, C. Mackay, R. Anders, B.
Greenwood, I. Clark, L. Miller, A. Fell and A.
Boyd for very useful discussion. We acknowledge research support from NH&MRC (Australia) and UNDP/World
Bank/WHO
Special Programme for Research and Training
in Tropical Disease.
References
1. Clark, I. A., Hunt, N. H., Butcher, G. A. and
Cowden, W. B. 1987. Inhibition of murine
malaria (Plasmodium chabaudi) in viuo by recom-
2.
3.
4.
5.
6.
7.
8.
9.
binant interferon-y or tumor necrosis factor,
and its enhancement by butyiated hydroxyanisolej. Immunol. 139: 3493-3496.
Currier, J., Sattabongkot, J., Rosenberg, R. and
Good, M. F. 1992. 'Natural' T cells responsive
to malaria: evidence implicating immunological cross-reactivity in the maintenance of
TCRa/P + malaria-specific responses from
non-exposed donors. Itu. Immunol. 4: 985994.
Goerlich, R., Hacker, G., Pfeffct. K., Heeg, K.
and Wagner, H. 1991. Plasmodium jakiparum
merozoites primarily stimulate Vy^ subset of
human y/5 T cells. Eur.J. Immunol. 21: 26132616.
Jones, K. R., Hickling, J. K., Targett, G. A. T.
and Playfair, J. H. L. 1990. Polyclonal. in vitro
proliferative responses from nonimmuiie donors to Plasmodium falciparum malaria antigens
require UCHLl + (memory) T cells. Eur. J.
Immunol. 20: 307-315.
Brown, J., Greenwood, B. M. and Terry, R. J.
1986. Cellular mechanisms involved in recovery from acute malaria in Gambian children.
Parasite Immunol. 8: 551-564.
Brake, D. A., Long, C. A. and Weidanz. W. P.
1988. Adoptive protection against Plasmodium
chabaudi adami malaria in athymic nude mice
by a cloned T cell line. / Immunol. 140:
1989-1993.
Good, M. F. 1991. The implications tor malaria vaccine programs if memory T cells from
non-exposed liumaas can respond to malaria
antigens. Curr. Opinion immunot. 3: 496-502.
Mackay, C. R. 1992. Migration pathways and
immunologic memory among T lymphocytes.
Sem. Immutiol. 4: 51-58.
Rodrigues, M., Nussenzweig, R. S., Romero,
P. and Zavala, F. 1992. The in vivo cytotoxic
410
M, F. Good and]. Currier
activity of CD8 * T cell clones correlates with
their levels of expression of adhesion molecules./ Exp. Med. 175; 895-905.
10. Greenwood, B. M., Bradley, A. K., Greenwood, A. M. et al. 1987. Mortality and morbidity from malaria among children in a rural area
of The Gambia, West Africa. Trans. R. Soc.
Trop. Med. Hyg. 81: 478-486.
11. Hill. A. V. S., Allsopp, C. E. M., Kwiatkowski.
D. et al. 1991. Common west African HLA
antigens are associated with protection from
severe malaria. Nature 352: 595-600.
12. Pongponratn, E., Riganti, M., Punpoowong,
B. and Aikawa, M. 1991. Microvascular
sequestration of parasitized erythrocytes in
human falciparum malaria: a pathological
study. Am.J. Trop. Med. Hyg. 44: 168-175.
13. Grau, G. E., Piguet, P-F,, Engers, H. D., Louis,
J. A., Vassalli, P. and Lambert, P-H. 1986.
L3T4 + T lymphocytes play a major role in
the pathogenesis of murine cerebral malaria.
/ Immunol. 137: 2348-2354.
14. Grau, G. E., Fajardo, L. F., Piguet, P-F., Allet,
B., Lambert. P-H. and Vassalli, P. 1987. Tumour necrosis factor (cachectin) as an essential
mediator in murine cerebral malaria. Science
237: 1210^1212.
15. Kwiatkowski. D. 1990. Tumour necrosis
factor, fever and fatality in falciparum malaria.
Immunol. Lett. 25: 213-216.
16. Clark, I. A.. Rockett, K. A. and Cowden, W.
B. 1991. Proposed link between cytokines.
nitric oxide and human cerebral malaria. Parasitol. Today 7: 205-207.
17. Clark. L A.. Gray. K. M.. Rockett, E. J. et al.
1992. Increased lyphotoxin in human malarial
serum, and the ability of this cytokine to
increase plasma interleukin-6 and cause hypoglycaemia in mice: implications for malarial
pathology. Trans. R. Soc. Trop. Med. Hyg.
(in press).
18. Macdonald, G. 1931. The mechanism of infection with malaria in children living under
endemic and hyperendemic conditions. Indian
J. Med. Research 18: 1347-1372.
19. Garnham. P. C. C. 1970. The roie of the
spleen in protozoal infections with special reference to splenectomy. Acta Tropica 27: 1-12.
20. Guazzi, M. and Grazi. S. 1963. Considerazioni
su un caso do malaria quartana recidivante
dopo 53 ani di latenza. Rex'ista di Malariologia
42: 55.
21. Weiss, L. 1990. The spleen in malaria: the role
of barrier cells./mmMMo/. Lett. 25: 165-172.
22. Kumar, S., Good M. F., Dontfraid. F., Vinetz,
J. M. and Miller, L. H. 1989. Interdependence
of CD4* T cells and malarial spleen in immunity to Plasmodium vimkei: relevance to vaccine
development./ Immunol. 143: 2017-2023.
23. Bate, C. A. W., Taverne, J.. Bootsma, H. J.
et al. 1992. Antibodies against phosphatidylinositol and inositol monophosphate specifically inhibit tumour necrosis factor induction
by malaria exoantigens. Immunol. 76: 35-41.
24. Grau, G. E., Del Giudice. G. and Lambert.
P-H. 1987. Host immune response and pathological expression in malaria: possible implications for malaria vaccines. Parasitol. 94: S123137.