Prions: Protein Aggregation and Infectious Diseases - Physiological ...

1128 ADRIANO AGUZZI AND ANNA MARIA CALELLA
ity (162, 385). For scrapie in sheep and goats, infectivity
was assessed in a broad range of tissues using experimentally
inoculated goats as donors and uninfected goats as
recipients (384), and both spleen and lymph nodes were
shown to transmit scrapie.
In the decades following these discoveries, our understanding
of the extraneural immune phase advanced
considerably, and an impressive collection of data has
accrued that provides clues about peripheral prion pathogenesis.
The normal prion protein was found to be consistently
expressed, albeit at moderate levels, in circulating
lymphocytes (91). Subsequently it was clearly shown
in a wealth of experimental paradigms that innate or
acquired deficiency of lymphocytes would impair peripheral
prion pathogenesis, whereas no aspects of pathogenesis
were affected by the presence or absence of lymphocytes
upon direct transmission of prions to the CNS (263,
291). Klein and colleagues (4, 267) pinpointed the lymphocyte
requirement to B cells. Interestingly, it was shown
that only B cells within the secondary lymphoid organs
accumulated prions in a PrP C -dependent fashion, while
circulating lymphocytes contained no detectable infectivity
(415). A series of bone marrow transfer experiments
demonstrated that peripheral prion pathogenesis required
the physical presence of B cells, but the expression of the
cellular prion protein by these cells is dispensable for
pathogenesis upon intraperitoneal infection in the mouse
scrapie model (11). Adoptive transfer of Prnp / bone
marrow to Prnp o/o -recipient mice did not suffice to restore
infectibility of Prnp-expressing brain grafts, indicating
that neuroinvasion was still defective (53), yet intraperitoneal
infection occurred efficiently even in B-celldeficient
hosts that had been engrafted with B cells from
Prnp knockout mice (268, 354).
Although the requirement for B cells appears to be
very stringent in most instances investigated, it has
emerged that not all strains of prions induce identical
patterns of peripheral pathogenesis, even when propagated
in the same, isogenic strain of host organism (452).
Another interesting discrepancy that remains to be
addressed concerns the actual nature of the cells that
replicate and accumulate prions in lymphoid organs. In
the RML paradigm, four series of rigorously controlled
experiments over 5 years (18, 243, 269, 400) unambiguously
reproduced the original observation by Blättler et
al. (53) that transfer of Prnp / bone marrow cells (or
fetal liver cells) to Prnp o/o mice restored accumulation
and replication of prions in spleen. In contrast, Brown et
al. (70) reported a diametrically opposite outcome of
similar experiments when mice were inoculated with prions
of the ME7 strain. This discrepancy may identify yet
another significant difference in the cellular tropism of
different prion strains.
It is important to note that the results of the Blättler
study do not necessarily indicate that lymphocytes are the
Physiol Rev VOL 89 OCTOBER 2009 www.prv.org
primary splenic repository of prions: in fact, other experiments
suggest that this is quite unlikely. Instead, bone
marrow transplantation may 1) transfer an ill-defined
population with the capability to replenish splenic stroma
and to replicate prions, or less probably, 2) donor-derived
PrP C expressing hematopoietic cells may confer prion
replication capability to recipient stroma by virtue of “GPI
painting,” i.e., the posttranslational cell-to-cell transfer of
glycophosphoinositol linked extracellular membrane proteins
(277). Some evidence might be accrued for either
possibility: stromal splenic follicular dendritic cells have
been described by some authors to possibly derive from
hematopoietic precursors, particularly when donors and
recipients were young (250, 486). Conversely, instances
have been described in which transfer of GPI-linked proteins
occurs in vivo with surprisingly high efficiency
(278). GPI painting has been described specifically for the
cellular prion protein (302).
While it has been known for a long time that specific
strains of prions may preferentially affect specific subsets
of neurons, the Blättler/Brown paradox may uncover an
analogous phenomenon in peripheral prion pathogenesis.
The latter question may be much more important than it
appears at face value, since the molecular and cellular
basis of peripheral tropism of prion strains is likely to be
directly linked to the potential danger of BSE in sheep
(77, 185, 249), as well the potential presence of vCJD
prions in human blood (6).
1. Follicular dendritic cells and prion replication
As mentioned above, prion infectivity rises very rapidly
(in a matter of days) in the spleens of intraperitoneally
infected mice. Although B lymphocytes are crucial for
neuroinvasion, all evidence indicates that most splenic
prion infectivity resides in a “stromal” fraction. Instead,
the apparent requirement for B lymphocytes in peripheral
pathogenesis is more likely to be derived, at least in part,
from indirect effects, including the provision of chemokines
or cytokines to cells that efficiently replicate prions
in peripheral regions of the host body (14).
Any potential profiteer from these B-lymphocyte-derived
signals would be localized in close proximity to the
B lymphocytes, be of stromal origin, and should also
display PrP C on its cell surface. FDCs fulfill each of these
criteria. FDCs support the formation and maintenance of
the lymphoid microarchitecture by expressing homeostatic
chemokines and have a role in antigen trapping and
capturing of immune complexes by Fc receptors. Identification
of FDC-specific genes is extremely important
and useful, to assess the contribution of FDCs to prion
pathogenesis. Recently, the antigen FDC-M1 has been
identified as Milk fat globule EGF factor 8 (Mfge8) (282).
Gene deletion experiments in mice have shown that
signaling by both TNF and lymphotoxins is required for