complementing established tumor-protective
actions of immune cells based on the expres-
sion of tumor-specific neoantigens ( 32 ). As a
transcriptional regulator, p53 has been impli-
cated in diverse aspects of immunity, includ-
ing tumor-related immune responses ( 33 ). The
same holds true for Rb, raising the interesting
question of the extent to which the roles of
these two tumor suppressors are p21 dependent.
Our comparative analysis of p21-OE with
p16-OE and p27-OE suggests that coordinated
cell-cycle arrest and transcriptional activation
of select genes is a common feature of CDK
inhibitors that act through Rb hypophospho-rylation. We observed substantial overlap be-
tween genes activated by the three inhibitors,
but many genes were also specifically induced.
On the surface, this argues against the idea
that Rb dephosphorylation is the underly-
ing driver. However, hypophosphorylated Rb
appears to be a collection of Rb molecules that
are monophosphorylated at one of 14 known
phosphorylation sites, with each variant having
a characteristic binding-partner profile and
a distinctive transcriptional output ( 34 ). It is
therefore conceivable that p21, p16, and p27
create distinctive collections of monophos-
phorylated Rb molecules, which, through
differences in transcription factor binding
preferences, activate distinct target genes,
thereby yielding secretomes of different
complexities and biological properties. Our
observation that p16-OE does not induce
CXCL14 explains why these cells were not
subject to immune clearance. Consistent with
this finding, cells undergoing senescence
through interferon-g–and tumor necrosis
factor–mediated induction of p16 are not sub-
ject to elimination ( 35 ).
In conclusion, we demonstrate that p21,
through Rb hypophosphorylation, induces the
macrophage attractant CXCL14 to place cells
under immunosurveillance as part of an im-
mediate early response to cellular stress that
also includes cell-cycle arrest. Our study high-
lights the role of CXCL14, but the PASP contains
several other immune-modulatory factors, in-
cluding interleukin-7 (IL-7) and IL-34. Both
these interleukins induce M1 macrophage dif-
ferentiation ( 36 , 37 ), a phenotypic change that
we show coincides with the recruitment of
cytotoxic T cells to p21-expressing hepatocytes.
A greater understanding of IL-7 and IL-34 and
other PASP factors in stress-related immu-
nosurveillance could inform innovative ther-
apeutic opportunities for the treatment of
cancers, including cancers lacking p53 activ-
ity. The observation that lowCXCL14expres-
sion correlates with poor clinical outcome in
various human cancers, including cervical,
colorectal, endometrial, and head and neck
cancers ( 38 ), underscores the relevance of such
future efforts.Materials and methods
Mouse strains
L-KRASG12Vmice were generated from KH2
ES (C57BL/6×129Sv) cells according to previ-
ously described methods ( 39 ) using a modified
pBS31vector. Briefly, the tetracycline-inducible
promoter and the SV40 polyA signal in the
pBS31were replaced by a CAG promoter-FRT-
loxP-flanked STOP cassette (LoxP-STOP-LoxP, L)
andWPRE-bGH-polyA(WPRE-pA) fromAi9
(Addgene, #22799), respectively. The FRT site
after the CAG promoter was deleted using site-
directed mutagenesis and a multiple cloning
site (MCS) was added betweenLandWPRE-pA.Sturmlechneret al.,Science 374 , eabb3420 (2021) 29 October 2021 7 of 15
4 2 8ns**Tom+
cell clusters/mm2Relative expressionp21 Cxcl14 Igfbp3 Adam15 Ssc5d App Fn1 Psapnsns*****ns
ns
ns ns ns ns ns*ns nsns** ******* ***
ns**nsAd28B CD FTom+ hepatocytes (%)d4 d12 d28Dying Tom+ hepatocytes (%)d4 d12 d28* nsnsnsns*nsns*nsns******nsns***
********ns***** ns***ETom+
hepatocytes joinedby1 CD3+cells (%)d4 Tom+(n=4)
d4 Tom+;KRASG12V(n=4)
d4 Tom+;KRASG12V;p21-KO (n=5)nsns
nsnsns***
***nsp21lo
p21lo
p21hi
p21lo
p21lo
p21lo
p21hi
p21loSingleCluster**Gp21lo
p21lo
p21hi
p21lo
p21lo
p21lo
p21hi
p21lons
nsnsns ns***
***nsd4 d12Tom+
hepatocytes joined
by1 iNOS+
cells (%)Tom+EdU+
hepatocytes (%)d4 d12024106801020300241561005102515200204060012354012354Tom DNAFig. 6. p21-dependent immunoclearance protects against oncogenic growth.(A) RT-qPCR on flow-sorted
Tom+hepatocytes. (B) Proportion of hepatocytes that are Tom+and appear healthy (not dying). (C) Quantification
of dying Tom+hepatocytes. (D) Quantification of Tom+hepatocytes joined by≥1 iNOS+cells. p21hi, cells with
elevated p21 staining; p21lo, cells with baseline or background level p21 staining. (E) As in (D) but for hepatocytes
with≥1 CD3e+cells. (F) Representative image and quantitation of Tom+hepatocyte clusters. (G) Proportion of
Tom+EdU+hepatocytes in- or outside Tom+clusters. Scale bar: 20mm(F).MiceusedwereonaC57BL/6×129Sv
mixed genetic background. Data represent means ± SEM. Experiments in the above panels were performed
once. ns, not significant. P< 0.05; P< 0.01; P< 0.001. Two-way ANOVA with Sidak’s correction
[(d12 and d28 in (B) and (C)], one-way ANOVA with Sidak’s correction [(A) and (D) to (F) and d4 in (B) and
(C)] or unpaired two-tailedttest (G).
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