diff --git a/lib/reducers/thread-activity-reducer.js b/lib/reducers/thread-activity-reducer.js
index 255c87313..092ee8e81 100644
--- a/lib/reducers/thread-activity-reducer.js
+++ b/lib/reducers/thread-activity-reducer.js
@@ -1,61 +1,75 @@
 // @flow
 
+import { messageStorePruneActionType } from '../actions/message-actions.js';
 import {
   deleteThreadActionTypes,
   leaveThreadActionTypes,
 } from '../actions/thread-actions.js';
 import {
   logOutActionTypes,
   deleteAccountActionTypes,
 } from '../actions/user-actions.js';
 import type { BaseAction } from '../types/redux-types.js';
 import type { ThreadActivityStore } from '../types/thread-activity-types.js';
 import { updateThreadLastNavigatedActionType } from '../types/thread-activity-types.js';
 import { updateTypes } from '../types/update-types-enum.js';
 import type { ClientUpdateInfo } from '../types/update-types.js';
 import { setNewSessionActionType } from '../utils/action-utils.js';
 
 function reduceThreadActivity(
   state: ThreadActivityStore,
   action: BaseAction,
 ): ThreadActivityStore {
   if (action.type === updateThreadLastNavigatedActionType) {
     const { threadID, time } = action.payload;
     const updatedThreadActivityStore = {
       ...state,
       [threadID]: {
         ...state[threadID],
         lastNavigatedTo: time,
       },
     };
     return updatedThreadActivityStore;
+  } else if (action.type === messageStorePruneActionType) {
+    const now = Date.now();
+    let updatedThreadActivityStore = { ...state };
+    for (const threadID: string of action.payload.threadIDs) {
+      updatedThreadActivityStore = {
+        ...updatedThreadActivityStore,
+        [threadID]: {
+          ...updatedThreadActivityStore[threadID],
+          lastPruned: now,
+        },
+      };
+    }
+    return updatedThreadActivityStore;
   } else if (
     action.type === leaveThreadActionTypes.success ||
     action.type === deleteThreadActionTypes.success
   ) {
     const { newUpdates } = action.payload.updatesResult;
     if (newUpdates.length === 0) {
       return state;
     }
 
     let updatedState = { ...state };
     for (const update: ClientUpdateInfo of newUpdates) {
       if (update.type === updateTypes.DELETE_THREAD) {
         const { [update.threadID]: _, ...stateSansRemovedThread } =
           updatedState;
         updatedState = stateSansRemovedThread;
       }
     }
     return updatedState;
   } else if (
     action.type === logOutActionTypes.success ||
     action.type === deleteAccountActionTypes.success ||
     (action.type === setNewSessionActionType &&
       action.payload.sessionChange.cookieInvalidated)
   ) {
     return {};
   }
   return state;
 }
 
 export { reduceThreadActivity };