UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

(Mark One)

| | ☑ | ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
| | - | ------------------------------------------------------------------------------------

For the fiscal year ended December 31, 2018
or

| | o | TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
| | - | ----------------------------------------------------------------------------------------

For the transition period from _____ to _____
Commission file number of the issuing entity: 333-193376-19

Central Index Key Number of the issuing entity:
0001643293

COMM 2015-LC21 Mortgage Trust

(exact name of the issuing entity as specified in
its charter)



Central Index Key Number of the depositor: 0001013454

Deutsche Mortgage & Asset Receiving Corporation

(exact name of the depositor as specified in its
charter)



Central Index Key Number of the sponsor: 0001541294

German American Capital Corporation

(exact name of the sponsor as specified in its charter)



Central Index Key Number of the sponsor: 0001558761

Cantor Commercial Real Estate Lending, L.P.

(exact name of the sponsor as specified in its charter)



Central Index Key Number of the sponsor: 0001541468

Ladder Capital Finance LLC

(exact name of the sponsor as specified in its charter)



Central Index Key Number of the sponsor: 0001089877

KeyBank National Association

(exact name of the sponsor as specified in its charter)



|New York(State or other jurisdiction ofincorporation or organization of
        the issuing entity) | 47-4300612
        47-4322711
        47-7135776
        (I.R.S. Employer
        Identification Numbers)
|--------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------

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)

Registrant’s telephone number, including
area code:

(212) 250-2500







Securities registered pursuant to Section 12(b) of the Act: None.

Securities registered pursuant to Section 12(g) of the Act: None.

Indicate by check mark if the registrant is a well-known seasoned
issuer, as defined in Rule 405 of the Securities Act. o Yes ☑
No

Indicate by check mark if the registrant is not required to file reports
pursuant to Section 13 or Section 15(d) of the Act. o Yes ☑
No

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 o No

Indicate by check mark whether the registrant has submitted electronically
and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule
405 of Regulation S-T (Section 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant
was required to submit and post such files).

Not applicable.

Indicate by check mark if disclosure of delinquent filers pursuant
to Item 405 of Regulation S-K (Section 229.405 of this chapter) is not contained herein, and will not be contained, to the best
of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form
10-K or any amendment to this Form 10-K.

Not applicable.

Indicate by check mark whether the registrant is a large accelerated
filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions
of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging
growth company” in Rule 12b-2 of the Exchange Act.



|Large accelerated filer                                                                                                                                                                                                                                    | o                                                | Accelerated filer         | o
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------ | ------------------------- | -
|Non-accelerated filer                                                                                                                                                                                                                                      | ☑  (Do not check if a smaller reporting company) | Smaller reporting company | o
|                                                                                                                                                                                                                                                           |                                                  | Emerging growth company   | o
|If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. | o                                               

Indicate by check mark whether the registrant is a shell company
(as defined in Rule 12b-2 of the Act). o Yes ☑No

State the aggregate market value of the voting and non-voting common
equity held by non-affiliates computed by reference to the price at which the common equity was last sold, or the average bid and
asked price of such common equity, as of the last business day of the registrant’s most recently completed second fiscal
quarter.

Not applicable.

Indicate by check mark whether the registrant has filed all documents
and reports required to be filed by Section 12, 13 or 15(d) of the Securities Exchange Act of 1934 subsequent to the distribution
of securities under a plan confirmed by a court. o Yes o
No

Not applicable.

Indicate the number of shares outstanding of each of the registrant’s
classes of common stock, as of the latest practicable date.

Not applicable.

DOCUMENTS INCORPORATED BY REFERENCE

List hereunder the following documents if incorporated by reference
and the Part of the Form 10-K (e.g., Part I, Part II, etc.) into which the document is incorporated: (1) Any annual report to security
holders; (2) Any proxy or information statement; and (3) Any prospectus filed pursuant to Rule 424(b) or (c) under the Securities
Act of 1933. The listed documents should be clearly described for identification purposes (e.g., annual report to security holders
for fiscal year ended December 24, 1980).

Not applicable.




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EXPLANATORY NOTES

The sole purpose of this Amendment No. 1 to our Annual Report on
Form 10-K for the fiscal year ended December 31, 2018 filed with the Securities and Exchange Commission on March 21, 2019 (the
“Original Form 10-K”) is to replace the report on assessment of compliance and the accountants’ attestation
report of Rialto Capital Advisors, LLC filed as exhibits to the Original Form 10-K under Exhibit 33 and Exhibit 34, respectively,
with the corresponding exhibits attached hereto. Except as described above, no other changes are being made to the Original Form
10-K.

Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934,
as amended, this Form 10-K/A also contains new certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which
are attached hereto.

PART IV

Item 15. Exhibits, Financial Statement Schedules

| | 31 | Rule 13a-14(d)/15d-14(d) Certifications.
| | -- | ----------------------------------------

| | 33 | Reports on assessment of compliance with servicing criteria for asset-backed securities.
| | -- | ----------------------------------------------------------------------------------------

| | 33.3 | Rialto Capital Advisors, LLC, as Special Servicer on and after July 5, 2018
| | ---- | ---------------------------------------------------------------------------

| | 34 | Attestation reports on assessment of compliance with servicing criteria for asset-backed securities.
| | -- | ----------------------------------------------------------------------------------------------------

| | 34.3 | Rialto Capital Advisors, LLC, as Special Servicer on and after July 5, 2018
| | ---- | ---------------------------------------------------------------------------

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) 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/ Helaine M. Kaplan

Helaine M. Kaplan, President

(senior officer in charge of securitization of the depositor)

Date: October 25, 2019

/s/ Natalie Grainger

Natalie Grainger, Director

Date: October 25, 2019



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