Lymph Node Dendritic Cells Protect CLL Cells Via Mcl-1

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[3051] Follicular Cells Protect CLL Cells from

Apoptosis through a Mcl-1 Dependent Mechanism.

Irene M. Pedersen, Shinichi Kitada, M. Zapata,

J. Kipps, Yong Sung Choi, ,

C. . The Burnham Institute, La Jolla, CA, USA;

Hematology/Oncology, UCSD, La Jolla, CA, USA; Alton

Ochsner Medical Foundation, New Orleans, LA, USA; ISIS

Pharmaceuticals, Carlsbad, CA, USA

CLL cells have defects in the apoptotic signaling

pathways and therefore accumulate in vivo. However,

when removed from the patient and cultured in-vitro,

these malignant cells rapidly undergo apoptosis.

Recent studies suggest that leukemia cell survival can

be influenced by interactions with non-leukemia cells

the microenvironment of lymph nodes, marrow, and other

tissues.

To model such cell-cell interactions in vitro, we

cultured freshly isolated CLL cells with a follicular

dendritic cell line (HK), which was established from

human tonsil. We found that CLL cells co-cultured with

HK-cells were protected from apoptosis, either

spontaneous or induced by treatment with anticancer

drugs. We found that cell-cell contact, and not

soluble factors secreted by HK, was needed for

HK-induced protection.

CLL cells co-cultured with a 3T3-cell line expressing

CD40-ligand (CD154) also could be protected

transiently from undergoing spontaneous apoptosis.

However, in contrast to the effects following

CD40-ligation, CLL cells were not induced to change

their surface expression of CD27, CD40, CD54, CD80 or

CD95, following co-culture with HK.

Furthermore, although neutralizing mAbs specific for

CD154 could inhibit the activity of CD154-expressing

3T3 cells on CLL cells, addition of such antibodies to

co-cultures to CLL cells and HK cells did not mitigate

the HK-induced protection from apoptsis. Examination

of the expression of several apoptosis-regulating

proteins revealed that HK co-culture induced

leukemia-cell up-regulation of the anti-apoptotic

Bcl-2 family protein Mcl-1, whereas CD40-ligation

apparently up-regulated expression Bcl-XL.

To examine whether expression of Mcl-1 was necessary

for HK cell-induced protection, we introduced Mcl-1

antisense (AS) oligonucleotides into CLL cells using

electroporation. This reduced leukemia cell expression

of Mcl-1 was assessed by immunoblot analysis.

Antisense-mediated inhibition of Mcl-1 expression

significantly reduced HK- induced protection against

apoptosis in the CLL cells, whereas control

oligonucleotides had no effect.

In summary, our data suggest that HK induces

protection against apoptosis in CLL cells at least in

part through up-regulation of Mcl-1, and that this

mechanism is distinct from that achieved via

CD40-ligation.

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