Selection vectors and methods of selecting eukaryotic host cells

1. A process for producing a polypeptide, comprising:

a) culturing a host cell under conditions that allow for the expression and secretion of the polypeptide, wherein the viability of said host cell is dependent on folate uptake, wherein the host cell comprises:

i) an introduced polynucleotide encoding a selectable marker, wherein the selectable marker is a mutated folate receptor alpha, and

ii) at least one introduced polynucleotide encoding the polypeptide, wherein said polypeptide is secreted from said host cell;

b) isolating the polypeptide from the cell culture medium; and

c) optionally processing the isolated polypeptide,

wherein the mutated folate receptor alpha consists of the amino acid sequence shown in SEQ ID NO:1, wherein alanine is substituted to leucine at amino acid position 49, and

wherein the host cell cultured in step a) is a host cell selected according to a method for selecting at least one host cell expressing a polypeptide, wherein said method comprises:

a. providing a plurality of host cells, wherein the viability of said plurality of host cells is dependent on folate uptake, and wherein said plurality of host cells comprises:

i) the introduced polynucleotide encoding the selectable marker, and

ii) the at least one introduced polynucleotide encoding a polypeptide;

b. culturing said plurality of host cells in a selective culture medium comprising folate at a concentration of 50 nM or less, which provides a stringent selective pressure on the host cell; and

c. obtaining at least one host cell expressing the polypeptide.

2. The process of claim 1 , wherein the host cell comprises an expression vector or combination of at least two expression vectors comprising:

a) an expression cassette comprising the polynucleotide encoding the mutated folate receptor alpha;

b) at least one expression cassette comprising the polynucleotide encoding the polypeptide; and

c) an expression cassette comprising a polynucleotide encoding a dihydrofolate reductase as selectable marker.

3. The process of claim 1 , wherein the host cell has one or more of the following characteristics:

a) it is a mammalian cell;

b) it is a rodent cell;

c) it is a CHO cell;

d) it expresses an endogenous folate receptor;

e) it comprises an introduced polynucleotide encoding a further selectable marker, wherein the further selectable marker is involved in folate metabolism, and wherein the further selectable marker optionally is a dihydrofolate reductase; and/or

f) the introduced polynucleotides are stably integrated into the genome.

4. The process of claim 1 , wherein the host cell comprises an expression vector or combination of at least two expression vectors comprising:

i) the introduced polynucleotide encoding the mutated folate receptor alpha as the selectable marker, and

ii) the at least one introduced polynucleotide encoding the polypeptide.

5. The process of claim 1 , wherein the method for selecting at least one host cell expressing a polypeptide has one or more of the following characteristics:

i) additional selection cycles comprising steps b. and c. are performed;

ii) after step c., the cells are cultured in a culture medium comprising a non-limiting concentration of folate and are then again cultured according to step b. and obtained according to step c;

iii) one or more additional selection steps are performed prior to and/or after performing step b. and/or c., wherein said one or more additional selection steps are selected from a flow cytometry based selection and a selection for one or more additional selectable markers introduced into the host cell;

iv) the host cells are stably transfected; and

v) the selected host cells recombinantly express and secrete the polypeptide, wherein the polypeptide is an immunoglobulin molecule.

6. The process of claim 1 , wherein the selectable marker allows a more stringent and faster selection of high producers than a selection system which uses the corresponding wild type folate receptor alpha as selectable marker.