PHA stimulation of PBMCs results in sticky cells - The replies

Michie, John, Dr <> jm5 at
Wed Jan 2 04:01:18 EST 2008

Thanks to all who responded. Once again proof that Flowers area caring, sharing community, just as I tell all the students attending our Flow Cytometry Courses. I have forwarded the replies to my colleague who will study them and try out your suggestions.

All the replies to date are posted below and a pdf file sent by Calman Prussin (Detection of Intracellular Cytokines by Flow Cytometry) is attached.


Best wishes to all for 2008, on a glorious Summer day in Cape Town

John Michie

   +27 21 9389 539

 +27 21 933 8886

   083 449 2225



On Dec 13, 2007, at 4:05 AM, Michie, John, Dr <jm5 at> wrote:

A colleague has the following problem:

After stimulation of PBMCs with PHA the cells become sticky. Trying to analyse on flow shows reduced numbers of cells compared to fresh.

Cause of stickiness?

Any help flowers?


Reply 1.

What is the cause I have never found out, but the phenomenon is experienced by many people. PHA is the worst stimulant if You want to use FACS for any analysis. Cells form clumps and it is quite difficult to get them all out of the well. I have managed to overcome some of the problem by incubating the cells after harvesting in RPMI 1640 medium containing 100 mg/ml N-acetylgalactosamine at 37 °C for 20 min, my volume for the incubation was 100 microliters per million cells. Cells are free from the clumps after this procedure and it is possible to count them in the haemocytometer, have never tried FACS after N-acetylgalactosamine - would imagine it can interfere with some surface proteins. Other stimulants like enterotoxin or concavalin-A do not cause this problem - PBMCs stay in nice suspention and FACS is no problem. anti-CD3 (+IL-2) can also be used but of course is more expensive. I would be grateful if You could let me know about other suggestions you recieve. Good luck Kate


Dr Katarzyna Ruckemann-Dziurdzińska

Katedra i Zakład Fizjopatologii

Akademia Medyczna w Gdańsku

ul. Dębinki 7

80-809 Gdańsk

tel. +48 58 3491511

fax. +48 58 3491510

kruck at


Reply 2.

Probably lots of cell death. Try adding 20ug/ml DNaseI to the media & washes. 

bunny at


Reply 3.

DO you have 1mM EDTA in the staining buffers?


Susan.Watson at


Reply 4.

PHA (phytohemagglutinin A) causes aggregation of red cells and also white blood cells to a considerable extent. PBMC is purified over Ficoll to primarily enrich mononuclear cells. We routinely culture PBMC with PHA and notice that a fraction (5 to 10%) will die by apoptosis within a day or two. However, if you want to analyze the cells by flow cytometry, it is easy to disrupt the clumps using a glass or disposable pasteur pipet. Pipet 3 or 4 times, collect the cells, centrifuge and resuspend in ca, Mg-free Dulbecco's PBS (Invitrogen) + 0.2% BSA to minimize clumping. Cells can be stained in this buffer (+ 0.01% sodium azide) on ice for 20 minutes, washed with the same buffer, fixed in 1% freshly prepared paraformaldehyde and analyzed without any problem.




S. Jayaraman, Ph.D.

Associate Professor of Surgery

University of Illinois at Chicago

909 South Wolcott Avenue-704E MSB-M/C 790

Chicago, IL 60612

Phone: 312-355-5133

Fax: 312-355-1497

anue2468 at


Reply 5.

I don't really stimulate my PBMCs with PHA, but I have observed that they get sticky and a lot of doublets/aggregates show up in the scatter gate. I reduced this problem by adding a final concentration of 2% fetal bovine serum and 0.53mM EDTA to the PBS. Also, keeping them cold/on ice/4C is important in reducing their stickiness. Hope this helps.

Nidal Muvarak

flowphilly at


Reply 6.

PHA is phyto-haemo-agglutinin, therefore is the cause of the stickiness. 

You can try washing your cells after stimulation and resuspending them a couple of times, or adding EDTA as is normally done for intracellular whole blood staining protocols (see BD website for a list of protocols). I don't know if the EDTA will change some expression in your stimulated cells though.




Mara Rocchi BVM&S, PhD

Flow Cytometry Manager

Moredun Research Institute

Pentlands Science Park

Bush Loan, Penicuik


Scotland, UK

Mara.Rocchi at


Reply 7.

You probably need to add a chelator, such as EDTA to reduce the  

stickiness.  It's the activation which causes it.  I also use filter  

topped tubes to reduce the clumps running through my cytometer, but  

you do lose some cells this way.


bmcbey at


Reply 8.

PHA causes severe cells agglutination and it is difficult if not impossible to break up the aggregates. PHA M for has stronger agglutinating properties than PHA P, but the latter also agglutinates the cells. The aggregates tend to disperse after 48 h of incubation. Pokeweed Mitogen (Phytolacca) is another polyvalent mitogen - it does not induce agglutination but it activates B cells. 

Zbigniew Darzynkiewicz, M.D., Ph.D. 
Professor of Pathology and Medicine 
Director, Brander Cancer Research Institute 
New York Medical College 
BSB, Room 438 
Valhalla, N.Y. 10595 <> 

Reply 9.
PHA causes some cell death which releases DNA which causes cells to stick, presumably via a charge-charge interaction. A 5 minute incubation with 3500 Dornase units/ml of DNAse (in a Mg++ containing buffer!) will liberate them.
DNAse details on page 6.24.4. [of attached pdf file]
cprussin at

Reply 10.
It is a natural phenomenon that PHA stimulation of PBMC results in cluster formation, as phytohemagglutinin agglutinates blood cells (according to its name). Moreover, polyclonal cell activation results in massive upregulation of adhesion receptors, thus these cells stay together after division and will also exercise increased stickiness during sample processing. Nevertheless, we have routinely used mitogen stimulated cells in flow analysis, so as many others, especially since inception of division tracking dyes. Some points for handling stimulated cells:

i) Careful pipetting while harvesting cells from culture plates is usually enough for disrupting the clusters. The effect can be controlled by microscopy or simply observing cells in wells in an inverted microscope. At the minimum, one should check the disrupting effect while establishing one's routine resuspending technique. In case of clinical diagnostic procedure, we specified the number of times for expelling the content of a well and pipette volume to standardize resuspension in the SOP.

ii) Make sure the sample is resuspended before acquisition on a flow cytometer, as the cells may also clump during staining and washing/centrifugation. This can also be checked in a microscope, especially if the flow picture casts any doubts.


iii) Doublet discrimination strategies have to be in place, so as exclusion of occasional "too large" events.


Merry Holidays and Happy New Year to every one!


Vladislav Rozenkov

rozenkov at


-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: warning1.txt
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: warning2.txt
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: warning3.txt
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: warning4.txt

More information about the Cytometry mailing list