Multilaboratory comparison of in vitro tests for chromosome aberrations in CHO and CHL cells tested under the same protocols.

Different test results have been reported for the same chemicals in two in vitro chromosome aberration test systems, CHL cells tested by a Japanese protocol and CHO cells tested by the US National Toxicology Program [Sofuni et al., Mutat Res 241:173-213,1990]. Here, laboratories in Japan, the US and the UK tested 9 such chemicals in CHL and CHO cells using the same protocols and found all 9 positive in both cell types; differences in earlier conclusions with these chemicals were due mainly to test protocol, not to different sensitivities of the cells. The most important protocol difference is sampling time. Chemicals that were negative in the NTP series using a sampling time of 10 to 13 hours often produced positive results when retested here with a 20- to 24-hour sampling time. While positive results were obtained in both cell types, CHL cells sometimes had higher aberration levels and survived at higher doses than CHO cells would tolerate. This may reflect some intrinsic difference in sensitivity but may also be affected by factors such as cell cycle length and culture media (e.g., oxygen scavenging capacity). The collaboration reported here also contributed to a better understanding of scoring aberrations, especially "gaps"; there was good agreement on what types of aberrations should be included in the totals when scoring criteria were clearly defined, for example, many changes classified as "gaps" by the Japanese system were classified as "breaks" in the scoring systems used in the United States and the United Kingdom, and were appropriately included in total aberration counts.