Parasites of the genus Schistosoma
likely arose over 70 million years ago 
and have undergone complex co-evolution with their definitive hosts, resulting in parasite adaptations that both evade and exploit host immune functions. Previous studies showed that schistosomes require host CD4+
T cells for normal development 
and to mediate egress of eggs from the body across the bowel wall 
. While egg excretion requires induction of an effector Th2 response to egg antigens 
, the CD4+
T cell effector functions that facilitate blood fluke development have not been elucidated. Because previous studies failed to identify a specific role for either Th1 or Th2 responses in schistosome development 
, the question of whether any CD4+
T effector functions are required to promote schistosome development remained unresolved. Therefore, the purpose of this study was to test whether antigen receptor signaling and subsequent activation of CD4+
T cells are necessary for normal parasite development to proceed. Our results show that, in contrast to the requirement for effector T cell responses to facilitate egg excretion, neither recognition of schistosome antigens nor TCR-mediated activation of CD4+
T cells are required for normal parasite development. Our findings suggest that none of the effector functions typically associated with CD4+
T cell responses are directly implicated in facilitating schistosome development, and may explain why previous attempts to identify a single T cell factor that modulates schistosome development, using knockout mice deficient in individual cytokines or their receptors 
, have been unsuccessful.
The schistosome requirement for CD4+
T cells, but the lack of necessity for traditional T cell effector functions, suggests that the steady-state homeostatic activities of naïve CD4+
T cells make the host environment more conducive to schistosome development. While the role of homeostatic interactions with MHCII+
cells in maintaining the peripheral CD4+
T cell pool is well established, the necessity of these same interactions for efficient APC maturation is being increasingly recognized 
. In T cell-deficient mice, APC development and function are compromised but can be restored by reconstitution of peripheral T cell populations 
. Furthermore, it was recently demonstrated that T cell conditioning of APCs for efficient maturation occurs under steady-state conditions before the initiation of T cell responses, in the absence of cognate antigen, and is mediated, at least in part, by the co-stimulatory molecule B7-H1 expressed by naïve T cells 
. We propose that similar steady-state interactions between naïve OT-II T cells and MHCII+
cells in OT-II/RAG-1-/-
mice account for the baseline increases in APC-related gene expression and the alterations in mononuclear cell maturation we observed in these mice.
Given the ability of naïve T cells to prime for efficient APC maturation, we hypothesized that exploitation of APC function by schistosomes, rather than of CD4+ T cells directly, accounts for the enhanced parasite development we observed in TCR-transgenic RAG-/- mice. While we cannot exclude the possibility that unknown factors elaborated by resting, naïve T cells directly influence schistosome development, our hypothesis provides a parsimonious explanation for why naïve CD4+ T cells that do not respond to schistosome infection can still influence parasite development. That schistosome development can be restored in the complete absence of CD4+ T cells, through direct maturation of APCs, supports this hypothesis by demonstrating that CD4+ T cells are not directly required for parasite development.
As the sentinels of the immune system, circulating monocytes and tissue macrophages and dendritic cells, also known as the mononuclear phagocyte system, specifically express pattern recognition receptors (PRRs), including TLRs, which allow for the detection of invading pathogens 
. Indeed, monocytes express high levels of TLRs and are the predominant producers of proinflammatory cytokines during endotoxic shock 
. Thus while other cells in RAG-/-
mice express PRRs, the monocytes, macrophages and dendritic cells are the predominant responders to pathogen-associated molecular patterns (PAMPs) such as LPS and are the likely mediators of the effect of LPS on schistosome development. Monocytes, macrophages and dendritic cells are ontologically related, as monocytes are the macrophage and dendritic cell precursors that migrate into the tissues, both during steady-state conditions and during infection. In response to infection, PRR ligation stimulates increased monocyte flux to meet the elevated demand for antigen-presenting and effector cells to combat infection 
. Our demonstration that LPS administration restores schistosome development is therefore further evidence in support of the hypothesis we propose above, that schistosome development is influenced by innate mononuclear cell function. Indeed, our data point to mononuclear cell function as the common mechanism underpinning the enhancement of schistosome development by both naïve CD4+
T cells and LPS. This hypothesis is under further investigation.
Several hypotheses can be proposed to explain how mononuclear phagocytes might influence shistosome development. Recruited to sites of infection and tissue damage, mononuclear cells produce cytokines and chemokines in response to activation that could act as cues for developing schistosomes 
. Alternatively, the localized release of proinflammatory cytokines by mononuclear cells would be predicted to increase blood flow and vascular permeability, perhaps increasing the supply of host factors required for normal parasite development. At sites of inflammation, monocyte-derived cells also contribute to blood vessel remodeling and angiogenesis, by secreting angiogenic factors and trans-differentiating into endothelial cells 
. As schistosomes are intravascular parasites and early parasite development occurs within portal venules, vessel remodeling may be required to allow for parasite growth. Yet another possibility is that mononuclear phagocytes may directly damage schistosomes, therefore requiring their local depletion for schistosome development to proceed normally. Activated macrophages and dendritic cells can produce nitric oxide 
, a molecule that is toxic to developing schistosomes 
and mediates immunity to schistosome infection in animals vaccinated with irradiated cercariae 
. Priming of APC maturation by CD4+
T cells or TLR ligands might allow for more rapid progression of these cells to apoptosis in response to activation. In RAG-/-
mice, developmental impairment of APCs might allow for their persistence, constituting a persistent source of nitric oxide and/or other molecules that impair schistosome development. If this is the case, analysis of innate responses to developing schistosomes in RAG-/-
mice may identify innate effector mechanisms that can be harnessed to enhance immunity to schistosome infection. These hypotheses are currently being tested.
Our data demonstrate that blood flukes do not respond to CD4+
T cells directly, but rather respond to signals that originate from the innate immune system. These findings corroborate previous studies where tumor necrosis factor (TNF; Mm.1293), an innate proinflammatory cytokine, was shown to stimulate parasite egg laying in CD4+
T cell-deficient Prkdcscid/scid
. While TNF does not appear to stimulate parasite development directly 
, the overlap between the TNF receptor and TLR signaling pathways could account for the ability of both ligands to enhance schistosome development 
. Our findings also have implications for understanding epidemiological associations between schistosomiasis and other infections 
, such as salmonellosis. While schistosome infection has been implicated in persistence of Salmonella
, our data also suggest that proinflammatory stimuli produced in response to bacterial LPS could play a role in exacerbating schistosome infection by supporting parasite development in co-infected individuals. These putative immunological and epidemiological associations are currently under investigation. The possible requirement of schistosomes for proinflammatory signals to support normal development is intriguing, as schistosomes, and helminths in general, are largely unable to stimulate such responses themselves due to a lack of potent TLR ligands 
. It is therefore tempting to speculate that, under evolutionary pressure to avoid immune detection, schistosomes have lost the ability to stimulate the inflammatory feedback required for their successful development and now rely on other mechanisms to generate these essential inflammatory signals.
In summary, our investigation of the mechanism by which CD4+
T cells facilitate schistosome development has revealed that blood flukes require neither CD4+
T cell responses nor associated effector functions. Furthermore, our data show that schistosomes do not respond directly to CD4+
T cells, as their requirement for these cells can be bypassed completely by direct stimulation of innate immune responses. Indeed, our data suggest that the role of CD4+
T cells in facilitating schistosome development may be limited to the provision of non-cognate T cell help for the maturation of MHCII+
APCs. We provide two lines of evidence in support of this hypothesis. First, non-responsive, naïve CD4+
T cells, which also condition immature APCs to undergo maturation, support improved parasite development. Second, direct stimulation of APC maturation in the absence of CD4+
T cells restores schistosome development. These data, together with previous findings that macrophages mediate vaccine-induced immunity to schistosome infection 
, implicate mononuclear cells as central host determinants of the outcome of schistosome infection in the definitive host. A detailed understanding of the interactions between blood flukes and the mononuclear phagocyte system could therefore identify opportunities to modulate mononuclear cell function in ways that impair or prevent the establishment of schistosome infections.