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).