UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington D.C. 20549

FORM 10-D

ASSET-BACKED ISSUER
DISTRIBUTION REPORT PURSUANT TO SECTION 13 OR 15(d) OF
THE SECURITIES EXCHANGE ACT OF 1934

For the monthly distribution period from:
AugustSeptember 13, 2025 to SeptemberOctober 1210, 2025

Commission File Number of issuing entity: 333-193376-19
Central Index Key Number of issuing entity: 0001643293

COMM 2015-LC21 Mortgage Trust
(Exact name of issuing entity as specified in its charter)

Commission File Number of depositor:  333-193376
Central Index Key Number of depositor: 0001013454

Deutsche Mortgage & Asset Receiving Corporation
(Exact name of depositor as specified in its charter)

Central Index Key Number of sponsor (if applicable): 0001541294
German American Capital Corporation
(Exact name of sponsor as specified in its charter)

Central Index Key Number of sponsor (if applicable): 0001558761
Cantor Commercial Real Estate Lending, L.P.
(Exact name of sponsor as specified in its charter)

Central Index Key Number of sponsor (if applicable): 0001541468
Ladder Capital Finance LLC
(Exact name of sponsor as specified in its charter)

Central Index Key Number of sponsor (if applicable): 0001089877
KeyBank National Association
(Exact name of sponsor as specified in its charter)

Lainie Kaye (212) 250-2500
(Name and telephone number, including area code, of the person to
contact in connection with this filing)

New York
(State or other jurisdiction of incorporation or organization
of the issuing entity)

Lower Tier Remic 47-4300612
Upper Tier Remic 47-4322711
Grantor Trust 47-7135776
(I.R.S. Employer Identification No.)

c/o Deutsche Bank Trust Company Americas as Certificate Administrator
1761 East St. Andrew Place, Santa Ana CA
(Address of principal executive offices of the issuing entity)

92705
(Zip Code)

(212) 250-2500
(Telephone number, including area code)

NONE
(Former name, former address, if changed since last report)

Registered / reporting pursuant to (check one)
Title of Class  Section 12(b)  Section 12(g)  Section 15(d)  Name of Exchange
(if Section 12(b))
Class A-1           [ ]             [ ]             [X]        Not Applicable
Class A-2           [ ]             [ ]             [X]        Not Applicable
Class A-SB          [ ]             [ ]             [X]        Not Applicable
Class A-3           [ ]             [ ]             [X]        Not Applicable
Class A-4           [ ]             [ ]             [X]        Not Applicable
Class X-A           [ ]             [ ]             [X]        Not Applicable
Class A-M           [ ]             [ ]             [X]        Not Applicable
Class B             [ ]             [ ]             [X]        Not Applicable
Class C             [ ]             [ ]             [X]        Not Applicable
Class D             [ ]             [ ]             [X]        Not Applicable

Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act
of 1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to
such filing requirements for the past 90 days. Yes _X_   No ____


Part I - DISTRIBUTION INFORMATION

Item 1.  Distribution and Pool Performance Information.
On  SeptemberOctober 1210, 2025 a distribution was made to holders of the
certificates issued by COMM 2015-LC21 Mortgage Trust.  The
distribution report is attached as Exhibit 99.1 to this Form 10-D.

During the distribution period from AugustSeptember 13, 2025 to SeptemberOctober 1210, 2025
no assets securitized by Deutsche Mortgage & Asset Receiving
Corporation (the "Depositor") and held by COMM 2015-LC21 Mortgage
Trust were the subject of a demand to repurchase or replace for breach
of the representations and warranties contained in the underlying
transaction documents.

The Depositor filed a Form ABS-15G pursuant to Rule 15Ga-1 under the
Securities Exchange Act of 1934 on February 13, 2025.  The CIK number
of the Depositor is 0001013454.

German American Capital Corporation ("GACC"), one of the sponsors and
mortgage loan sellers, filed a Form ABS-15G pursuant to Rule 15Ga-1
under the Securities Exchange Act of 1934 on February 13, 2025.  The
CIK number of GACC is 0001541294.

Cantor Commercial Real Estate Lending, L.P. ("CCRE"), one of the
sponsors and mortgage loan sellers, filed a Form ABS-15G pursuant to
Rule 15Ga-1 under the Securities Exchange Act of 1934 on
AugustOctober 0717, 2025.  The CIK number for CCRE is 0001558761.

Ladder Capital Finance LLC ("Ladder"), one of the sponsors and mortgage
loan sellers, filed a Form ABS-15G pursuant to Rule 15Ga-1 under the
Securities Exchange Act of 1934 on February 14, 2025.  The CIK number
of Ladder is 0001541468.

KeyBank National Association ("KeyBank"), one of the sponsors and
mortgage loan sellers, filed a Form ABS-15G pursuant
to Rule 15Ga-1 under the Securities Exchange Act of 1934 on
August 14, 2025.  The CIK number of KeyBank is 0001089877.


Part II - OTHER INFORMATION

Item 10. Exhibits.
(a) The following is a list of documents filed as part
    of this Report on Form 10-D:

(99.1)   Monthly Report distributed to holders of the
         certificates issued by COMM 2015-LC21 Mortgage Trust,
         relating to the SeptemberOctober 1210, 2025 distribution.

(b) The exhibits required to be filed by the Registrant
    pursuant to this Form are listed above.


SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

Deutsche Mortgage & Asset Receiving Corporation
(Depositor)


/s/ R. Chris Jones
Name:  R. Chris Jones
Title: Managing Director


/s/ Matt Smith
Name:  Matt Smith
Title: Director



Date:    SeptemberOctober 1920, 2025
Filing Compare · COMM 2015-LC21 Mortgage Trust
