The balance between life and death in plasma cells
di Caterina Fulgione
Preface
The humoral response can be considered the highest
expression of immune response, and so it is ruled by
fundamental mechanisms.
After the encounter with APCs (Antigen Presenting Cells), B
cells terminally differentiate in plasma cells. A portion of
them survives for a prolonged periods to mantain long
term humoral immunity, while most of them starts to
secrete antibodies with a rate of 10000 per second for each
cell.
Discussion
Apoptosis usually needs, in the intrinsic pathway, the loss
of the mitochondrial transmembrane potential (Δψm),
caused by proteins of Bcl2 family, and the consequent
release of mitochondrial protein in the cell soup, activing
the apoptosome and the effector caspases (caspase-3 and
9).
Immunoblotting results. It’s clear that the amount of cleavage
products of caspase-3, caspase-9 and Mst1, a target of caspase-3
involved in apoptotic chromatin compaction, is higher in resting
B-cell cultures in spite of the substantially lower rate of
apoptosis in the B-cell cultures (<20%) compared with the
plasma cell cultures (40%)(fig.1B PMID 20651073)
The three cellular cultures in analysis undergo apoptosis
after few days since differentiation and the consequent
massive antibody secretion according to different times.
Apoptosis is usually induced by DNA or plasma membrane
damages, but it is proved that malfunction of others
organelle can have the same effect. The Endoplasmatic
Reticulum (ER) is a cellular compartment that can lead to
cell death. An unbearable ER stress can lead to
programmed cell death through caspase-dependent
pathways when UPR, Unfolded Protein Response, cannot
cope the store of misfolded Ig and sends intrinsic apoptotic
signals. It is also possible that cellular compartment stress
induce cell death through a caspase-indipendent pathway.
Some experiments led on primary plasma cells, in vitro
differentiated plasma cells and plasmacytic I.29µ+ cells,
differentiated using interleukins and lipopolysaccharide
(LPS), shown that the immunoblotting (an electrophoretic
techinique based on a protein/antibody correspondence)
of these cellular cultures displays a low level (or absence)
of active caspases-3 and 9. B resting cells, taken in analysis
as a control reference, present a higher level of these
immunoblotting products, despite a lower tendency to
apoptosis.
Apoptotic rate of the cellular cultures with relation to the flow of
the days. It is interesting to notice that I.29µ+ cells start
apoptosis at day four since LPS somministration, that confer
them most key characteristicsoif plasma cells but not the full
postmitotic state. Primary plasma cells undergo apoptosis after
two days in culture, as in-vitro differentiated plasma cells do
(fig.1A PMID 20651073).
The treatment with the pan-caspase inhibitor zVADfmk
had no effect on the apoptotic rate of the three cellular
cultures in analysis. The treatment of resting B-cells with
zVADfmk, instead, increased the apoptotic rate of these
cells. It has ben supposed that plasma cell death is caspaseindipendent and the effector caspase block happens during
the diffentiation, when the cell develops the compartments
needed for the secretion and the triaspartic acid repeat
that has to be cut to active effector is stabilized. During this
development of the secretory pathway , the proteosome
does not improve its capacity, but, intriguingly, the capacity
of the proteasome to degrade misfolded proteins
decreases during plasma cell differentiation. That is a
surprinsing detection, because it would be expected that, if
the secretion and the relative amount of misfolded Ig
increase, an enlarged degradation capacity would be
requested. That find has been considered part of a balance
between the necessity of an efficient secretive capacity and
the need of a strictly controlled humoral response.
The rise of ER stress does not induce apoptosis with the
celerity of the caspase-dipendent pathway, but it also
cause a situation that does not allow the cell to keep living.
The cell reach a level of ER stress, that can be tunamycininduced, after which the ER dependent caspase-4 (12 in
mice), responsible of nuclear apoptosis, is expressed, and
two Bcl2 proteins, Bim e Bax, induce a non reversible
change of Δψm. At this point, apoptosis happens according
to mechanisms caspase-indipendent that are not clearly
understood.
The same experiments had been carried out on nonlymphoid cells, as MEFs (fibroblast embryonic murine). A
MEF cellular culture was double knock out (DKO) for
caspase-3 and caspase-9 (the MEFs where deprived of both
alleles for effector caspases), while another one was
heterozygote. The results of DKO culture were the same of
plasmacytic differentiated I.29µ+, while the heterozygote
culture underwent apoptosis through caspase-dipendent
pathways. The caspase-indipendent apoptotic mechanisms
can be considered common to others cellular types, too.
Conclusion
The delay caused by the caspase-block, which is mostly due
to the triaspartic acid stabilization on the cleavage site of
caspases 3 and 9, allows the cell to keep secreting in a
condition that would have usually been critical, and
maintaining an high rate of Ig secretion. On the other side,
the restricted proteosome capacity does not allow to
dispose of misfolded Ig, determining an unbearable long
term cellular environment. All these device are useful to
induce cell death after the accomplishment of its function.
The short-lived plasma cell life span is regulated to permit
an effective humoral immune response, that has to face a
danger for the system and be strictly defined in time.
A further knowledge of short-lived plasma cell apoptosis
has clinical implication: mice lacking the proapoptotic
protein Bim show a tendency to autoimmune kidneys
disease, and both murine and human myeloma have a
block of the caspase-dipendent apoptotic pathways, that
could be reactivated as a treatment for that disease.
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