All cell lines are first tested for mycoplasma infection. Mycoplasma is a common contaminant of cultured cell lines, and, if present, can adversely affect the growth rate and antibody secretion rate of the cells and can also make it difficult to expand cell cultures into large growth vessels. If a mycoplasma infected cell line is grown in a laboratory, it is very difficult to avoid cross contamination into previously uninfected cultures. If a cell line is mycoplasma positive, it undergoes a regime of treatment, developed in the laboratory, with the antibiotic ciprofloxacin. This is usually effective in eliminating mycoplasma from cell cultures.
Treatment continues until the culture gives a negative result with the mycoplasma test.
Once a treated line is mycoplasma test negative, normal work continues on the line, but the line is retested at later stages of growth, since mycoplasma infection can sometimes recur. Stability of cell lines is monitored by limiting dilution cloning analysis 2 . All lines are cloned at least three times after primary fusion, and cloning continues until all clones isolated are secreting specific antibody. The best clones are then selected - i.e., those showing optimal growth characteristics, together with a high level of secretion of specific antibody that has the highest potency for the lowest antibody concentration. We have observed significant variation in antibody avidity between different sister clones 4 .