Separate attention must be given to susceptibility of tissues of different species to the virus in tissue-culture systems. In monolayer cultures, Stewart and Eddy were first to report propagation of polyoma virus in monkey-kidney cells (33) and then in mouse-embryo cultures (22). Cytopathogenic effects were not noted by them in these reports, but were later reported in trypsinized, monolayer cultures of whole mouse embryos (34). Rabson and Legallais (27) observed viral multiplication and cytopathogenic effect on milk-medium cultures of an altered murine lymphoma cell line, P388 D(. Dawe, Law, and Dunn (IS) reported similar findings, using the same cell line grown in a human serum medium, and reported cytopathogenic effects in two other hemic tumor cell lines of mice, P329 (35) and the MBIII cell line of De Bruyn (36). It has also been found possible to establish carrier cultures of P388 D1 cells infected with the agent (13). Dmochowski (37) reported growth of a hamster-adapted strain of polyoma virus on human amnion cells as well as on rat-embryo cells. Dulbecco (S8) recently noted a proliferative response by infected cultures of hamster cells, not accompanied by any cytolytic response.

Morphologic observations on the effects of polyoma virus upon intact fragments of salivary-gland tissue of the newborn mouse in vitro have been described by Dawe and Law (39), who found proliferative as well as cytocidal effects side by side in the same cultures. These effects were noted as early as 14 days after exposure of the tissues to virus, but were more pronounced after periods of 19 to 48 days. This experiment has subsequently been repeated in an identical manner, with the same virus pool and the same gelatin sponge matrix method of culture but substituting human fetal tissues for the salivary-gland tissue of newborn mice (40). Two fetuses serving as tissue donors were at about the 6th and 8th months of development. The tissues used were from the parotid, submaxillary, thyroid, and thymus glands. A total of 18 infected and 18 control cultures was studied by serial sections after periods up to 30 days of incubation. Survival of the salivary glands was good, but no recognizable cell response, either cytolytic or proliferative, was observed. For reasons to appear later in this paper, more extensive trials with human tissues should be made with this and other systems. Similar tests for reactivity of salivary glands of the rat, hamster, and Mastomys to polyoma virus are in progress in this laboratory.

Information on possible differences among different species of mice is lacking. Rowe (SO) found that "wild" mice (Mus muscvlus) trapped from certain geographic areas showed positive serologic reactions against polyoma virus, whereas wild mice from other areas were serologically negative. The responsiveness of wild mice in terms of tumor development is not known.

Information as to host-strain differences of susceptibility is limited entirely to the mouse. This area requires further exploration, because recognition of distinct strain differences in susceptibility would offer opportunity for analysis of genetic influences. The mechanism of action of genetic influences could then be sought in differences of immune response, differences in timing of embryologic development, differences in endocrine factors, or constitutional intracellular differences of the host cells concerned.

Thus far, every inbred strain of laboratory mouse adequately tested has been shown capable of responding to polyoma virus with tumor development in at least one organ, the parotid gland. Positively responding strains include: AKR, C3H/An (44), C3Hf/He (5, 45), C3Hf/Lw (IS, 46), C3H/BiGs (19), C3H/BiLw (45-47), C3H/Fg (13), C58/Lw (13), C57BL/Ka (13), C57BL/Ha (42), C57BR/Lw (13), DBA/1 (42), DBA/2 (13), RFM (13, 48), StOLI/Lw (13), SWR (18), Swiss/ IcrHa (42). In addition to these inbred strains, noninbred Swiss mice and hybrid (C3H/f X AKR)F1 mice were among the first to be shown responsive to this agent (5, 21). The (AKR X Swiss) F1 cross also responds (42) as do (C3Hf/Lw X AKR)F2 mice (49).

At least three strains have given evidence of partial resistance: C57BL/Ka (13), C3H/Lw (44, 45), and C57BR (13, 60). In an early report of Gross (50), no tumor response was observed in strain C57BR. However, when highly active tissue-culture preparations of virus were used (13) it was found that one of 6 C57BR animals developed parotid tumors after receiving a dose that produced tumors in more than 90 percent of mice of other strains. The apparent relative resistance of this strain has not been further studied. Preliminary evidence on the relative resistance of C57BL/Ka mice has been presented (18) and is included in table 1. Not shown in the table is that among the 3 C57BL/Ka animals that developed tumors, 1 had unilateral parotid tumors only, 1 had bilateral parotid tumors only, and the 3d had a thymic tumor and bilateral parotid tumors only. It was of further interest that the parotid tumors in all 3 were unusually uniform histologically, all having a solid, "medullary" pattern without glandlike structures or the pleomorphism generally found in these tumors. Among the 39 tumor-bearing mice of other strains, there were 27 with thymic tumors, 17 with small thyroid tumors, 12 with bone tumors, 7 with hair-follicle tumors, and 5 with mammary tumors.

Table 1. Tumor Response To Polyoma Virus* By Strain C57bl/Ka As Compared With 5 Other Inbred Lines

Strain

Number of mice

Number of mice surviving 1 month

Number of mice with parotid tumor

Number of litters (positive/ total)

Latent period (months)

DBA/2Lw

16

11

11

2/2

2. 9

STOLI

14

4

3

1/1

3. 7

AKR/Lw

21

10

10

2/2

1. 4

RFM

14

13

11

2/2

1. 9

C58/Lw

5

5

4

1/1

2. 5

Total for

strains other

than C57BL/Ka

70

43

39 (91%)

8/8

-

C57BL/Ka

91

41

3 (7%)

2/7

3.5

*All animals received 0.05 ml. of culture-fluid pool from first passage on P388 Di cells, diluted 100:1, when less than 24 hours old.

Plaque techniques have been applied to the polyoma virus (51-64) and the results of two of these studies (52, 58) indicate that only a single agent is concerned in the induction of multiple tumor types. On these grounds, it is evident that relative resistance may be manifested by quantal response differences, by a smaller variety of tumor types induced, and by an increased latent period required for the appearance of tumor. This renders the evaluation of dose-response effects of polyoma virus at once immensely interesting and mathematically intricate. The system is probably the most complex one available for application of the principles reviewed by Bryan (55) for the quantitation of tumor viruses. As yet an ideal study correlating particle counts, hemagglutination titer, mouse infectivity, tissue-culture infectivity, and oncogenic response has not been reported. A positive correlation between particle counts and hemagglutination titer has been found by Kabler et al. (56) for one strain of the agent.