The HALO® Page

HALO® is the acronym for Hematopoietic/Hematotoxicity Assays via Luminescence Output. HALO® is the flagship assay platform from which all other standardized ATP bioluminescence assays for different biological systems were derived.

Why use ATP bioluminescence as the readout for all HALO® assays? Because it is the most accurate and sensitive readout system for rare cell populations such as hematopoietic stem and progenitor cells. No other assay readout comes close.

HALO® was originally developed because of the need for a high-throughput hematopoietic stem and progenitor cell toxicity screening platform for the biopharmaceutical industry. With the help of a SBIR grant from the National Cancer Institute (NCI), the traditional colony-forming unit (CFU) or cell (CFC) assay, first published in 1966, was rebuilt from the ground up into a standardized and validated, methylcellulose proliferation and differentiation assay that is now called CAMEO™-96. CAMEO™-96 remains to this day, the only standardized CFU assay available. 

With the development of Suspension Expansion Culture™ (SEC™) Technology that obviated the use of methylcellulose, the next generation of CAMEO™-96 became the HALO® Platform. The incorporation of SEC™ Technology coupled with the biochemical measurement of intracellular ATP (iATP) using luciferin/luciferase bioluminescence technology, allowed the transformation of the traditional CFU assay into the most advanced hematotoxicity stem cell screening and testing platforms available.

Originally developed just for in vitro human toxicity testing, the HALO® Platform has been further developed to include non-human primate, horse, sheep, pig, minipig, dog, rat and mouse cells, thereby allowing the response of new drug candidates and other compounds to be directly compared in different species.

Although rare stem cell populations such as the colony-forming cell granulocyte, erythroid, macrophage, megakaryocyte (CFC-GEMM) can be identified in the methylcellulose CFU assay, the latter is extremely insensitive to detecting rare stem cell populations. It is one of the reasons why detection of the CFC-GEMM population has never been routinely performed as a means to qualify stem cell quality or potency prior to hematopoietic stem cell transplantation.

HALO®, on the other hand, is many times more accurate and sensitive than the CFU assay in detecting the rarest hematopoietic stem cell populations. As a result, HALO® can be used to detect and quantitatively measure up to 11 different stem cell populations and 12 different lympho-hematopoietic progenitor cell populations using low serum and/or serum-free culture conditions for different species.

For decades, the CFU assay was the only in vitro, hematopoietic growth assay available. It provided many important breakthroughs in hematopoietic research. However, at the same time, investigators complained about the disadvatages and flaws of the assay. Indeed, it is one of those flaws, namely assay reproducibility, that remains the greatest challenge. It fact, it is safe to say that many of the publications using the CFU assay are probably not scientifically reproducible. This is due to the lack of standards, controls and the inability to validate the assay. For this reason alone, the methylcellulose CFU assay is an obsolete assay in modern scientific research and other applications.  
While Preferred Cell Systems™ still produces 2 traditional CFU assays namely, ColonyGro™ and CAMEO™-4, and other methylcellulose CFU reagents (e.g. MethoCult) are produced by competitors, they pale in comparison to any of the assays in the HALO® Platform. Regardless of the specific application, each HALO® assay contains the controls and standards needed to calibrate and standardize the assay. You, the investigator, can rest assured that when you purchase any HALO® assay kit, it will work reliably and reproducibly, right out of the box. You can also rely on the fact that you will be working with an assay that demonstrates the greatest accuracy, sensitivity, selectivity and the highest precision of any assay yet developed for hematopoietic cells.

The HALO® Platform has been designed for 3 primary application areas:
  1. Stem cell and basic hematopoietic research,
  2. Hematopoietic cellular therapy,
  3. In vitro hematotoxicity high-throughput screening and testing.
All HALO® assay kits include and are for 96-well plates. For large numbers of samples and for screening, assay kits are also avilable with 384-well plates upon request.

The following is a complete list of all the HALO® assays available for the different application areas. Clicking on the assay link will take you directly to the specific webpage. From there, you can also go to our online ordering catalog to see the different assays available (usually for different cell populations and species) and their cost.

1. Stem Cell and Basic Hematopoietic Research Applications
  • CAMEO™-96: The only standardized CFU assay available that detects both cell proliferation within colonies and allows colonies to be differentiated into differnt colony types.
  • HALO®: The research assay available for individual stem and progenitor cell populations for multiple species. All assay kits include low serum Master Mixes and serum-free reagents are available for some species.
  • HALO® "Global" are research assays that allow 4-, 5- or 7-populations to be determined simultaneously, side-by-side.
2. Hematopoietic Cellular Therapy Applications
  • HALO® PCA: Progenitor Cell Assays the complete replacement for, equivalent to and directly correlated with MethoCult™reagents H4434, H4034, H4534, H4035 and H4435.
  • STEMpredict™: A 3-day predictive stem cell "quality" designed for all three stem cell sources, but specifically for umbilical cord blood and mobilized peripheral blood samples to differentiate high from low quality samples prior to or after cryopreservation.
  • HALO® QC: A 5 day predictive stem cell quality control assay to ensure the presence and yield of stem cells before and after any preparation procedure, e.g. fractionation, cryopreservation. There is a FDA Master File for this assay. 
  • HALO® SC-IPS: A 7-day, FDA compliant stem cell potency assay for umbilical cord blood, bone marrow or mobilized peripheral blood samples. There is a FDA Master File for this assay.
  • HALO® TE: To determine neutrophil, platelet and/or erythroid time to engraftment after transplantation. The assay is completed in 5 days.
  • HALO® PMT "Global": Assessment of long-term hematopoietic and/or lympho-hematopoietic reconstitution after stem cell transplantation.
3. In Vitro Hematotoxicity High-Throughput Screening and Testing
  • HALO®-Tox HT: To determine the response of individual cell populations derived from different species to new drug candidates or virtually any other compound or perturbation. Designed for routine high-throughput screening in 96- or 384-well plates.
  • HALO®-Tox HT "Global". Same as HALO®-Tox HT, but for studying the response of 4-, 5- or 7-cell populations simultaneously.

HALO® laid the growndwork for developing other ATP bioluminescence assays for research, cellular therapy and regenerative medicine and in vitro toxicity testing for the following cell types:
  • Immune cells
  • Mesenchymal stem cells
  • Heart cells
  • Intestinal epithelial cells
  • Hepatocytes
  • Renal cells
  • Neural cells
  • Skin cells
  • ES, iPS and other stem cell types
  • Tumor cells
  • Cell lines

We invite you to investigate the Preferred Cell Systems website to find what you need to advance your application.

For more information, contact Preferred Cell Systems at or call 1 (719) 264-6251.