Discussion
This work shows that LL particles can elicit cell recruitment. We do not
know how the dose used in this work compares with the amounts of
particles accumulating locally in the infection setting. However, since
this infection can last for years or decades (limited by the host’s
lifespan), high local doses of LL particles must eventually be reached
unless the materials find their way into systemic circulation and are
captured elsewhere in the body. Circulation of LL materials and their
capture by Kupffer cells do take place, on the basis of our results in
intraperitoneal mouse infections (12). Therefore, our present results
suggest that at least part of the evolutionary advantage of LL materials
being mopped up efficiently by Kupffer cells may be to avoid the
excessive build-up of the materials and consequent cell recruitment to
the parasite’s vicinity. Complementarily, it is possible that a certain
level of host cell recruitment is acceptable or beneficial to the
parasite, especially if the recruited cells are induced to adopt
suppressive phenotypes. Indeed, cells recruited to the parasite’s
vicinity in intraperitoneal mouse infections display predominantly
suppressive phenotypes (27); the possible role of LL particles in this
phenomenon is under study in our group.
It is noteworthy that the calcium InsP 6component, which represents 1/3 of the mass of the LL, makes no
contribution to cell recruitment, and it actually tends to diminish it.
In terms of the generation of the major complement anaphylatoxin C5a by
LL particles exposed to normal human serum, calcium
InsP 6 has no net effect (15), although the
analogous experiment in mouse serum has not been carried out. The
present results suggest that in the mouse tissue milieu in vivo ,
calcium InsP 6 also has no net effect or has a
weak inhibitory effect on cell recruitment in general.
LL particles elicited cellular influx and resident macrophage
disappearance even if their mucin glycans were oxidised so as not to be
able to engage carbohydrate receptors (15, 16). In possible agreement,
out of a panel of mammalian innate immune lectin receptors, only Clec4F
(which in rodents is expressed only in Kupffer cells (35, 36)) bound
clearly to the LL mucins (13). The elicitation of host responses by the
LL independently of its major molecular-level motifs
(InsP 6 and mucin glycans, as well as a predicted
major exposed peptide sequence) was previously observed by our group
during in vitro work with DC (16, 18). Together with other
results, this led us to propose that LL particles may impact on DC and
other innate cells (except by Kupffer cells) by receptor-independent
mechanisms (3, 18, 25). The present results are in broad agreement with
this previous proposal.
The diminished capacity to recruit neutrophils (and possibly additional
cell types) shown by the periodate-treated material may be explained by
natural anti-carbohydrate antibodies binding the LL mucins and causing
some degree of classical complement pathway (CCP) activation. In broad
agreement, we previously observed that LL particles (free of calcium
InsP 6 and of infection-induced antibodies) cause
some CCP activation in normal human serum that must be ascribed to
natural antibodies (15). The LL mucin glycans may play a stronger role
in promoting cell recruitment in the infection context, in which they
are targets of specific antibodies that may further activate the CCP (3,
15).