The invention originates from a process for the quantitative optical analysis of fluorescently labelled biological cells which are in contact with a fluorescent dye solution or of luminescent cells which are applied to a transparent support at the bottom of a reaction vessel in the form of a coherent cell layer, or alternatively of fluorescently or luminescently labelled reaction components in a solution in which a fluorescent or luminescent ligand is dissolved, the solution being in contact with a receptor layer, which is specific for this ligand and situated on the transparent support at the bottom of the reaction vessel, whose fluorescent or luminescent radiation, which is characteristic of the receptor-ligand binding, is detected and analysed through the transparent bottom.
A problem in fluorescence measurement in biomedical assays is often that the fluorescence changes correlated with the biological cell action are small compared with the non-specific background fluorescence. As a result, the resolving power is greatly restricted. Conventional commercial measuring systems (fluorescence readers, Dynatech or SLT), cannot solve the problem, because owing to their optical measuring arrangement (excitation from ‘above’ through the fluorescent liquid column of the supernatant) the signal can barely be detected in comparison with the background. Apparatuses of newer construction (Labsystems), which illuminate the cells from the back through the transparent support of the reaction vessel, do have the advantage that on entry of the excitation light the cells are excited to fluorescence. Since the excitation light, however, enters further into the supernatant, which is also fluorescent, the fact that the non-specific background signal adulterates the cell signal cannot be avoided. Even very complicated measuring systems (NovelTech, FLIPR: Fluorescence Imaging Plate Reader) are only able to decrease this background fluorescence using a special laser illumination geometry (excitation below about 45░). The reason for the failure of all problem-solving experiments on the measuring geometry is the fact that the actual cause of the background fluorescence cannot be decisively influenced hereby.
In the receptor binding studies carried out until now using fluorescently or luminescently labelled ligands, the labelled and unbound fraction in each case must be removed by processes like washing. Many coatings, however, are sensitive to these washing steps. Moreover, the removal of the unbound ligand is associated with a considerable outlay. The direct measurement of the receptor-ligand association or dissociation is not possible in this process.
The invention is based on the object of improving the sensitivity of the optical analysis of fluorescently labelled or luminescent cells in a cellular assay in order to be able to measure, for example, membrane potential changes which are as low as possible on the basis of fluorescence changes of potential-sensitive dyes. In this case, the sensitivity of the measuring system should be so high that potential changes of below 5 mV can be detected at least qualitatively. In the case of luminescent cells, an increase in the detection of the luminescence signal should be achieved. Moreover, the method should be suitable for screening with a high sample throughput.
The invention is furthermore based on the object of simplifying receptor binding studies based on fluorescently or luminescently labelled ligands or receptors and making possible continuous measurement of the receptor binding interaction (kinetics). Owing to the reduction in the process steps necessary, this method should be particularly suitable for screening with a high throughput and for diagnostic applications.
It was only possible to achieve the required high resolution with low membrane potential changes after it was possible to eliminate the cause of the interfering overlapping of the non-specific background fluorescence and the specific fluoresence of the cells. The process according to the invention developed for this purpose is based on the fundamentally new idea of masking the excitation energy and the fluorescence not originating from the biological object. To do this, in addition to the fluorescent dye, a further dye is added which completely absorbs the excitation light of the fluorescent dye and/or its emission light without affecting the fluorescence of the cells. By means of this absorption, the non-specific background signal is masked and the useful cell signal can be detected with a resolution which was previously not possible.
An alternative solution which is within the scope of the invention is that a separating layer, which is permeable to the solution and which absorbs and/or reflects the excitation light for the fluorescent dye and/or its emission light without adversely affecting the cell properties, is applied to the cell layer. At the same time, the thickness of the separating layer is selected such that fluorescence is no longer detectable in the dissolving mixture with the fluorescent dye but without the cells.
A further variant of the invention is that the method of the separating layer according to the invention is also used for increasing the sensitivity in the quantitative optical analysis of luminescent (luminous) biological cells which are applied to a transparent support in the form of a coherent cell layer. For this purpose, the optical properties of the separating layer permeable to the solution are selected such that it reflects the luminescent light as strongly as possible without adversely affecting the cell properties. In this manner, it is possible to increase the luminescence intensity and thus the measured effect considerably.
The process according to the invention can be used in a completely analogous manner for the quantitative optical analysis of fluorescently or luminescently labelled reaction components in a reaction vessel filled with a solution, the fluorescent or luminescent ligand being present in dissolved form and the solution being in contact with a receptor layer which is specific for this ligand, applied to a transparent support at the bottom of the reaction vessel or deposited thereon, whose fluorescent or luminescent radiation, which is characteristic for receptor-ligand binding, is detected and analysed through the transparent bottom. In this case, the solution according to the invention of the object described above is based on the fact that the free ligand which is in the supernatant, i.e. in solution, and its non-specific fluorescence or luminescence is masked by an additional dye and/or by a diffusely absorbing or reflecting separating layer and thus the cause of the interfering overlapping of the non-specific background fluorescence and the specific fluorescence of the ligand in the solution is eliminated. Since the nonbound ligand is masked in this manner, the measured fluorescence or luminescence is a direct measure of the ligand-receptor interaction. It can be measured directly in this process with time resolution.
In receptor studies, in analogy to the process described above, the invention thus relates to a process variant in which a masking dye is added to the solution and/or a separating layer permeable to the solution is applied to the receptor layer, the optical properties of the masking dye and/or of the separating layer being selected such that the excitation light for the fluorescent dye of the ligand present in the solution and/or its emission light or its luminescent light is absorbed by the solution or the separating layer or reflected at the separating layer. In this case, the thickness of the separating layer is selected such that fluorescence is no longer detectable in the dissolving mixture with the fluorescent dye, but without the receptor layer.
The separating layer preferably consists of polymeric latex beads (e.g. polystyrene, polyurethane, butadiene, acrylonitrile). The latex beads can also be dyed with a masking dye, which in this case must have an adequately high polymer dyeing capacity.
In the first-mentioned process, the masking dye should be as well distributed as possible in the solution which also contains the fluorescent dye in dissolved form. Since, as a rule, the solvent is water, a masking dye is expediently employed which possesses good water solubility (>2 g/ml) and has no cytotoxic side effects.
According to a further development of the invention, after the replacement of the supernatant containing a fluorescent dye by a fluorescent dye-free solution, a further masking dye is added which suppresses a non-specific fluorescence on the reaction vessel wall.
The following advantages are achieved using the invention:
The new process described is not tied to a certain measuring system, but can be used, because it is not a specifically technical solution, by many commercially available apparatuses. These include virtually all fluorescence readers which can illuminate and also measure transparent reaction vessels, e.g. microtitre plates, from the bottom. With a very low outlay (minimal additional costs only for the special absorption dyes), it is possible for the first time by this means to advance in a resolving area, e.g. in the measurement of potential changes in cell membranes by measurement of the change in fluorescence of potential-sensitive fluorescent dyes, which was unachieved until now. For the first time it is possible even in the case of very low changes to carry out a direct comparison of the results from various reaction vessels (e.g. various wells in a microtitre plate), such that the complicated procedure of the determination of the relative change in a reaction vessel can be dispensed with. As a result the number of measurements to be determined, e.g. for kinetic measurements, decreases. The outlay in terms of time for a measuring programme is markedly reduced and the possibility created of obtaining identical results by a simple individual measurement (e.g. end point determination) with the use of reference to a separate control batch. The uniformity of the biological batch required in this case (e.g. homogeneous cell layer) is generally afforded, for example, for microtitre plates.
Surprisingly, the use of various water-soluble dyes and also their mixtures in the very different cells tested showed no negative effect on the physiology of the cells (e.g. reaction of the cells in comparison with electrophysiological measurements such as whole-cell patch-clamp, or effects of the pharmaceuticals investigated). The use of undissolved dye pigments or inorganic finely divided particles was also surprisingly well tolerated by the biological objects.
As a result of the simple process described for masking the background fluorescence in quantitative fluorescence measurement in biomedical assays, connected with an increase in the sensitivity, e.g. when using potential-sensitive fluorescent dyes, and the adaptability of this process, e.g. to microtitre plates as reaction vessels, the use of such measuring techniques will significantly simplify high-throughput screening, especially no increased technical expenditure is necessary for the realization of the advantages outlined, but existing commercial measuring apparatuses are sufficient for this purpose. In receptor-ligand studies, the advantage essential to the invention is that, on account of the masking of the non-specific fluorescence or luminescence, it is no longer necessary to remove the unbound fraction of the ligands. As a result, the test procedures are considerably simplified, damage to and destruction of the sensitive coatings or of the biological objects such as, for example, cells are avoided and the sensitivity and thus also the accuracy of the measurement are improved. As a result of the use of microparticles, the utilizable surface area for the coating of fluorescently or luminescently labelled ligands can be significantly increased. By means of suitable measures, e.g. relatively high specific density or the use of magnetizable particles, the settlement and concentration of the microparticles on the transparent support can be achieved. In this case too, the fluorescence or luminescence of the unbound ligands in the supernatant is effectively suppressed by the masking.
Since the interaction between the ligands and the receptor must not be interrupted by the removal of the unbound fraction, a continuous measurement of the interaction between ligand and receptor (kinetics) can be carried out in this manner even in an individual reaction batch.